If a person is tall, which adjectives can I use? Which one of them is most common in classical Latin? The most suitable-looking adjectives I know are altus, procerus and longus, but I found no clear indication as to which ones can be used to describe people. For example, if my friend is two meters tall, which adjectives work if I say amicus meus duo metra altus/procerus/longus/… est?
All three of those adjectives are used of persons in classical Latin, in both prose and poetry. (In my own reading, though, I'm accustomed to seeing altus and procerus much more often in this context than longus.)
(Definitions and attestations are from Oxford Latin Dictionary.)
1 Having great extension upwards, lofty, tall
... (of persons or animals) tanta vis inerat in verbis...ut altior fieres (Cic. Fin. 2.51); cunctis altior ibat Anchises (Verg. A. 8.162); altior illis ipsa dea est (Ov. Met. 3.181) ...
3 (usu. of persons) Having (great) upward extent, tall
... longus an brevis...sit (homo) (Cic. Inv. 1.35); Quintia...candida, longa, recta est (Catul. 86.1); (Ov. Am. 5.3.8); qui mendacio staturam adiuvant longioresque quam sunt, videri volunt (Sen. Ep. 111.3) ...
1...b (of persons) tall
... b forti nubet procera Corano filia Nasicae (Hor. S. 2.5.64); procera corpora, promissae...comae (Liv. 38.17.3) ...
As for the specification of measurement, the entries for both altus and longus show examples that use an accusative, genitive, and ablative; however, none of those examples involve persons – unless you count artistic representations of human/divine forms, for which only the entry for altus provides examples, and only for accusative and ablative:
Lysippi Iuppiter...quadraginta cubita altus (Lucil. 526) ... signum septem pedes altum (Liv. 8.10.12)
colossus altus pedibus XC (Hyg. Fab. 223.3)
Still, for your example, it seems to me as though you'd be justified in saying either amicus meus duobus metris altus est or amicus meus duo metra altus est.
I suspect that the reason OLD doesn't provide examples that involve persons is that the Romans themselves would tend not to be too concerned about the precise measurement of persons, because this info wouldn't have very much practical value, as opposed to, say, the exact height of city wall, rampart, or other fortification if you're on military campaign, or the exact height of a column if you're constructing a building, monument, or aqueduct. The exact height of a horse would have some practical value too. Instead, for persons, they would use generalizations such as 'tall'/'too tall'/'very tall' or 'short'/'too short'/'very short,' and leave it at that.