Hairstyles, body types, skin tones, age range, et cetera.
Full disclosure: I am not a historian. I am a reference librarian, and the number of upvotes and lack of comments intrigued me, so I decided to go digging.
I had a hard time finding specifics about what was considered attractive; it may be because there wasn't a standard of "beauty" but rather the idea that beauty was more of a spiritual thing. From Edler:
During the thousand years that followed the Roman Empire, artists moved away from depictions of ‘ideal’ human form as the concept of physical beauty gradually became unacceptable as an ‘ideal’ in an increasingly religious atmosphere. Physical beauty in other words gave way to ‘spiritual’ beauty (Peck and Peck, 1993).
I believe more on this spiritual beauty can be found in the book The Mirror of the Self: Sexuality, Self-Knowledge, and the Gaze in the Early Roman Empire by Shadi Bartsch.
It seems like there were a few periods of using math to explain beauty. Again, Edler:
During the Renaissance, however, the Roman/Greek guidelines for beauty again came under close examination. Figure 5a⇓ may well represent Da Vinci's precursor to the subsequent ‘rule of fifths‘, dividing the ideal face transversally into five equal eye widths. Figure 6a⇓ illustrates Da Vinci's work on proportions within the face as compared with Figure 6b⇓, namely some of Durer's proposed arrangements for ideal proportions. Amongst these, the relationship between nose and ear length is identical to Da Vinci's.
Peck and Peck - http://www.angle.org/doi/pdf/10.1043/0003-3219(1970)040%3C0284:ACOFE%3E2.0.CO%3B2 - have an article that covers a sort of brief history of attractiveness. There isn't a section on the Roman Empire, but there is a "Greeks" and "After the Greeks." From the Greek section (and I include this only because it is similar to the above information regarding beauty and the spiritual, and also beauty and math):
The philosophers felt that beautiful creations respected certain geometrical laws, since true beauty necessarily displayed harmony. As harmony was the "due observance of proportions," it seemed reasonable to assume that these proportions were fixed quantities. Of course, "aesthetics" as introduced by the Greeks and expounded by modern philosophers encompasses more than simple physical nature or beauty. The beauties of human emotion, existence, and experience are manifestly important to the aestheticism.
It also seems like there was a strong connection between strength and beauty. From Gardiner's Athletics in the Ancient World:
The ideal cannot be found in any extreme of type, in strength or beauty by itself, but only in a combination of the two. His athletic training had taught the sculptor the value of physical strength, systematically trained and developed; his artistic sense taught him that no subject was worthy of his art which did not present beauty of line and proportion. hence that union of strength and beauty which characterizes the art of this period.
My apologies if this isn't the right kind of answer. It seemed like you were going for "long hair is considered most beautiful," or "pale bodies are most attractive." This is what I was able to find though, and I hope it at least encourages you to keep looking!
edited for formatting
Finally, something I can help with. The Roman poet Catullus wrote a poem (Carmen 43) comparing an unnamed woman with his quasi-fictional lover Lesbia. Full text:
Salve, nec minimo puella naso nec bello pede nec nigris ocellis nec longis digitis nec ore sicco nec sane nimis elegante lingua, decoctoris amica Formiani. ten provincia narrat esse bellam? tecum Lesbia nostra comparatur? o saeclum insapiens et infacetum!
Which translates as:
Greetings, girl without the smallest nose, nor pretty feet, nor dark eyes, nor long fingers nor a dry mouth nor a tongue (speech) much refined, companion of the bankrupt Formianus. Does the country call you beautiful? Is our Lesbia compared with you? Oh what a stupid and tasteless age!
From this, we can get a general form of the things Romans of the Late Republic may have found desirable in women.
This is, of course, from Catullus' perspective. But it can probably be used as a rough idea for "attractiveness" for Roman nobles in the Late Republic.
Sources:
http://rudy.negenborn.net/catullus/text2/e43.htm (for Catullus translation) http://rudy.negenborn.net/catullus/text2/l43.htm (Latin text)
Some of the most photorealistic portraits to emerge from the Roman Empire were the Fayum "mummy portraits." They were nevertheless paintings, designed to subtly flatter the subject by exaggerating certain features.
These, along with marble sculptures of the same period, reveal that Roman ideas of beauty weren't that far from modern ones. Young men and women are both typically painted with large eyes; long straight noses; and plump, delicately curved lips. Wavy hair (common in the Mediterranean) is shown to its best advantage, in tight curls or cascading ringlets. The idealized human body in Rome borrowed heavily from classical Greek sculpture. The male body is slim and athletic, with sharply defined muscles. The female body is curvy and lush, with full hips and smaller, firm breasts. (The unclothed female form is relatively uncommon in Roman sculpture, except where it is directly copying Greek sculpture.)
While there are a lot of good answers here, I'd also include Ovid's Art of Love as well; he gives an idea of not only aesthetic beauty, but also the types of personalities which are thought as attractive as well as the sneaky things you can do to get close to a woman (as well as a warning to women to be on the lookout for creepers who use his tactics). You'll be surprised how little has changed in 2000 years.
You might also want to check out the frescos (don't quote me on them being proper frescoes -- they're painted on plaster but I won't swear that the process is the same) from the Villa of Mysteries in Pompeii.
In these we see visible musculature on the male and a woman being flogged from behind, both of which are posed to show off aspects of their physique.
