GS300 speaker upgrade... Where to start?

[Prof Prolapse]

Hi,

I've got an 2005 Lexus GS300 and I'm looking to upgrade the sound system but don't know where to start. I'm also constrained by the multifunctional head unit.

I can do most things automotive but don't know much about this sort of thing, presumably just wildly changing speakers to the best that will fit in the gap will be of limited value?

Or will it?