From wikipedia:
In the early Norse sources, there is no mention of their being short; in the legendary sagas, however, they are "small and usually ugly". Anatoly Liberman suggests that dwarfs may have originally been thought of as lesser supernatural beings, which became literal smallness after Christianization.[10] Old Norse dwarf names include Fullangr ('tall enough') and Hár ('high'), whereas Anglo-Saxon glosses use dweorg to render Latin terms such as nanus and pygmaeus ('pygmy').
On a probably less relevant note, the "Makers" from the PC game Darksiders 2 fits quite well to the physical descriptions suggested by the wikipedia: very tall, old beards, old-looking, skilled smiths and crafters, gatekeepers, and as their titles suggests, responsible for building and maintaining the world somehow. I mention that because it seems plausible the game creators found something mentioning those mythological entities as taller than humans.
The problem with answering your question (having been summoned by /u/grandpohbah) is that Nordic mythic dwarves weren't actually anything. I don't mean to burst your bubble (SPOILER ALERT), but since they didn't actually exist, there is no clear way to describe what they were in any definitive way.
Belief systems are enormously flexible, particularly when they are not tethered to a written text. Beliefs were different from place to place and time to time, so no one can claims with any degree of credibility to have the definitive description of any entity along the lines of a Nordic dwarf.
On top of that, size is an extremely flexible thing when it comes to Northern European supernatural beings. There are all sorts of stories about elves, dwarves, dragons, and even giants, where they are characterized with a full range of possibilities.
The following is from my forthcoming book on Cornish folklore (still in the process of being edited). This not about dwarfs or Scandinavia, but the discussion about relative sizes is pertinent to this discussion here:
Besides the Cornish, many other people thought of fairies as being small. The Irish famously refer to them as the ‘wee folk’, and for the Danish they are ‘pysslingar’, which also refers to their small size. At the same time, tradition typically maintained that small beings could become larger and could greet people in ways that made them appear to be normal human beings. The principal distinction between one culture and another is whether pre-industrial people thought of supernatural beings as spending most of their time in a diminutive state. In Norway, Iceland, and Sweden, they were generally human-sized. While the Irish thought of the fairies as capable of being little, they did not see them as consistently small in the way that the Cornish and Danes did. Cornish legends attest to the idea that whilst fairies were generally about knee-high, they could shrink to even smaller sizes. And as William Bottrell records in ‘The Fairy Dwelling on Selena Moor’, there was at least some belief that each time a fairy changed into the form of an animal, it became smaller.
The petite nature of piskies inhibited stories about romance between the two worlds, but the difference in size could be overcome when the supernatural being took a larger form. There were other occasions when Cornish piskies assumed human size. For example, in the legend of a midwife or nurse summoned by a stranger, the women realizes he is unusual only when she sees him at a market. The narrative, which is widespread in Northern Europe, consistently ends with the astonishing discovery that the husband of the woman giving birth is in fact invisible to everyone but her. Before that final incident, there was nothing about the father, the mother, or their baby, whether size or some other feature, that could distinguish them as peculiar. This shows how amorphous belief and tradition could be: even in areas where indigenous fairies were generally believed to be small or hideous in some way, it was believed that they could also transform to appear like normal people. There are also Cornish legends that portray piskies growing into gigantic stature. (Sources: Coincidentally, Lotte Motz addresses the issue of relative size regarding giants, which could appear large but also small: ‘Giants in Folklore and Mythology: A New Approach’, Folklore, 93:1 (1982), pp. 70-84, especially pp. 73-74.)
The point here is that people who believed in these things had stories that could depict them with a full range of sizes.
I suspect if we were fortunate enough to have a group of tenth-century Nordic speakers in the room with us (and we were all able to converse), we could ask them your question, and the resulting discussion would be lively. One of them would tell a story about how they were fairly small, but then would answer that he had heard another story where they were human-sized. Another would respond that once she heard of a dwarf who was fairly small, but then he grew much larger until he became gigantic. This discussion would reveal that they could not necessarily agree on details, and then they would shake their heads in agreement about the enegmatic nature of the stories they had heard, and that while they didn't always agree with one another, the stories nevertheless reveal the strange nature of the otherworld and all its inhabitants.
There is a wonderful scene from the film, 'The Thirteenth Warrior" when they bring in an older man who had been attacked by one of the bear warriors. They ask him for details, and he says something along the line of, 'well they were sort of like that, and sometimes they weren't.' (I'm sure some helpful redditor can quote this more accurately). The point is in the ambiguity and contradictory nature of these early belief systems.
So in answer to your question, 'Were the Nordic mythical dwarves actually very tall?'
The answer is, some say they were, and some say they weren't. It's as though they were both at the same time. Or could become small and then tall at a whim.
This thread hits closely to what you are asking: https://www.reddit.com/r/AskHistorians/comments/4x7tzw/where_did_the_lore_of_trolls_elves_dwarfs_etc/
u/itsallfolklore is the pro on this topic. If you want some great reads about folklore (as should be obvious from the username) read through their post history.