The map? Well, I don't think I reached the max but I didn't want more in it because I already had more than 7 civs to cover. including Italy and North-Africa might have been a problem in several ways. Accuaracy for example.
Besides I wanted to concentrate on the region you can see there now because that's were all the important events happened.
Trebizond wasn't really set out to be played. But it can be done! I tried that ,however, if you don't succed to built up a good economic and military base chances are you will fail in achieving a lot of progress.
There are quite a few units that have the marine capability. Trapezuntines don't have any because they really were militarily insignificant and both land and sea. Also it slows down the Venetians a bit, you're right.
There were several reasons for not allowing dips/spies. First I wanted to use them but discovered I could not disable the steal tech option for the AI controlled dips/spies.
That continuesly messed up the different tech trees for the civs since all civs have a part only they can research. Units as you might see can and will become obsolete but I couldn't use NO techs because I needed a tech tree worth researching.
Second I needed the unit slots of both the dip and spy really badly. They were the last I filled because I was hoping to find a way around the problem.
Third I noticed that due to the fact that one can make quite some money through trade routes some cities were made revolt that I didn't want to revolt. That, of course, favoured civs that had a lot of money. Meaning Venice could have won easily just by buying other civs' cities and that wouldn't have been much fun at all for its opponent.
It's nice to get some good feedback from you!