It's actually not too unreasonable for each tagma to be at the most 1,500 men strong. The emperor Diocletian broke up the Roman legions into smaller units 1,000 men strong, so it's possible that the tagmata could have followed this precedent. The idea behind the reorganisation was that smaller legions would be easier to control and reinforce. The Rhine and Danube legions were prone to revolting, so they were split up and redeployed to other areas.
I actually came across a website yesterday (I forget the URL) which had a passage from a Byzantine account that described the Scholai as being no more than "eighteen banda" strong. The cavalry banda were only 50-100 men strong (Compared to the 200-400 strong infantry banda), so the Scholai would have been 1,800 men strong at the most, eventually rising to 3,000 men by 1066. The Excubitors were the same size as the Scholai (Because they were created as a tagmata alongside it), so you'd have 3,000 men in Constantine V's time, which is similar to the 3,500 strong Scholae that Constantine created to replace the Praetorians.
If you add in the other two tagmata, the Hikanatoi and Arithmos (Which never changed size), you'd come out with a maximum of 6,000 men by 867, and 9,000 by 1066. If you include the Varangian Guard (Which was also considered a tagma), you get a maximum of 12,000 men in 867 and 15,000 in 1066. If you combine retinue changes and the vanilla mercenaries (The Great Company is probably as large as the Byzantine Companions got) you can easily field 20,000 men. I'll be giving Alexius I his Archontopoulai (A special regiment of 2,000 cataphracts made up of officer's orphans), so you won't be short of cavalry. The tagmata will be gone by 1081, but you'll be able to restore them for 500 gold and prestige. My only issue is coming up with a Roman name for the tagmata and the Archontopoulai.