Whats the size of a 1998 Toyota 4Runner stock speakers?

The front speakers are 6.25" they can fit 6.75" and below.
Possible fabrication needed may be needed. The rear door are actually 5" they do not make a 5" speaker so you have to get either a very slim mount 5.25" or use the 4".

Try to use Toyota speaker adapter plugs so you don't need to cut wires and mess the electric system up. You can get them from Ebay or Amazon.