Just how big can my score be at the maximum?

In FORTE Premium you can set up to 32 instruments per system.
Each instrument can have up to 8 voice layers.

The number of instruments per system and voice layers per instrument or track are reduced in FORTE Home and Basic (16/4 and 4/2). You can find a detailed overview here:

Product information

The score can be created up to DIN A3 (consider the possibilities of your printer).
Each document (*.fnf file) can contain a maximum of 1032 bars.