The problem with aftermarket radios and avalon isnt the space. There's a lot of realestate behind the panel and with a little bit of customizing you can give it a factory look and install almost any aftermarket.
The real problem is the factory radio's wiring that also controls the air conditioning. If you remove the factory radio, A/C doesnt work and if not mistaken also the blue screen wont either. I think someone posted a how to using a custom made or altered harness to connect and leave the original radio in the trunk while using an aftermarket.