what is the biggest size of speakers that fit into the front dash?

Stock speakers are 4x10" size. I'm not sure that you could fit anything much bigger than that without some modding. Anyone done anything bigger?

do you think 6 by 9 would work

If they are shallow enough with a small enough magnet, and you bolt them in from the bottom, maybe. The real question is, just how much do you want to go through to install them? Almost anything "can" be done, it's just the time, effort and what you would have to sacrafice to do it that matters.

https://www.fiero.nl/forum/Forum2/HTML/081643.html

I used this to help with the installation of Infinity Kappa 50.9CS (5.25 component system) in the dash. Although I think the woofer of the 50.9 is slightly larger than that of the 50.7 because I had to make extensive modifications to the template provided in the post. I never got the chance to listen to pioneer 4x10 that came with the car, but even though I had only the left side set up, it still sounded nice. I couldn't wait to get the other side installed but my motor went out so I had to turn my attention to that