Imma going to stop right here fam. That was always a question I got when I was working in an 18th century history museum, and I hate to say it, but it has been spread by decades of poor scholarship and condescension towards the past. Nutrition, disease and environmental factors have a huge effect on human height, but we can say that if any individual were allowed to have a healthy diet, they were likely to reach close to modern day averages.

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The work of John Komlos, Willfred Fann, Kenneth L. Sokoloff, Georgia E. Villaflor, Matthew Spring and Kabbittskrieg have done wonders to offer some detailed data on average male height in the 18th century. Granted, these are very narrowly focused on male soldiers but it does offer us a window into height, with good data sample sizes too! This comes from Kabbittskrieg:

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British Regulars in the American War of Independence (sample size 1462): 65.77 inches

It should be noted that a large part of this sample (roughly 2/3rds) comes from the Royal Marines, which did not prioritize enlistment based on height to the same degree that the British army did. Therefore, this sample should not be taken as a definitive measurement of British soldiers' heights. The average for only army soldiers is 68 inches.

American Provincials in the French and Indian War (sample size 3057): 67.55 inches
Sample contains mostly men from New York.

American Continentals in the American War of Independence (sample size 5092): 68.1 inches
Sample contains mostly men from Virginia and Massachusetts.

Prussian Infantry in 1783 (sample size 3749): 69 inches

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Keep in mind that the average height for males in the US today is 69.3inches. So, not too far off. However, I cannot speak as to the height of other countries throughout time. Because, not to be pedantic, you asked quite the big and broad question.

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Sources:
Komlos, "On the Biological Standard of Living of Eighteenth-Century Americans"

Fann,  "Foreigners in the Prussian Army 1713-1783"

Sokoloff and Villaflor, "The Early Achievement of Modern Stature in America"

Spring, "With Zeal and With Bayonets"