I need your advice on the level pack i'm making. What do you think would be a good size for most of the levels, and are there any tilesets you think I should use? (other than what I have already chosen - heaven, egypt, swamps of the sleeping jaguar, and mez03a.)

You need to give us more information. BATTLE, SINGLE PLAYER or something else? Good sizes vary on type of level.

(On a last note, if you're going to be using all of Agama's tilesets, you're forgetting Sirius/Dimension)

First of all it is single player, and second, I took a look at sirius and it didn't seem to fit with the theme of my pack.

Personally, I Usually go with the default 256x64. Good enough for JJ1, good enough for me.

Well, you might want to use JAWS' Amazon Forest right before or after the Sleeping Jaguar Swamps.

Ok, the default seemed kind of small so I went with 400 x 100.