theoretically there is no limit - but since a sprite of 1000x2000 pixels wouldn't fit the screen, the practical size limit is probably somewhere around 500x400 or so...
There are no templates, because ANY size can be used - Ace calculates the sprite size based on the spritesheet size.
1) Select the size of the sprite you want (for example, let's say you want a 96x128 pixel sprite
2) Determine wether you want a single sprite or eight sprites on a single sheet (usually larger-sized sprites use single-sprite-sheets)
3) for single sprite sheets, multiply the X-size by 3 (for three animations) and the Y-Size by 4 (for four directions) to get the sprite sheet size: 96x3=288; 128x4=512.
4) create a spritesheet in the size of 288x512 and save it with a name that begins with ! (for example, "!largesprite.png")
5) Fill the spritesheets with the 12 sprite variants (3x4) for the direction and the animation with GIMP, Photoshop or similiar programs.
6) Import the result into Ace with the resource manager