How big can an .htaccess file be?
 

Can I keep adding to it indefinately?

I don't know if there is or what what size it would be, but you should keep it as short as possible, especially on high traffic sites, because Apache reads & processes it every time a document is requested. If it is getting too large you should probably start placing some things in your config file.

keep it as small as you can

how long is a piece of string?