There is a file called "config.txt" as in "/boot/config.txt" ....
Modify it to your needs, according to the documentation .
You might want to read this
Since i am using "remote desktop" to the RPI, i use these settings in my config.txt for a 720p resolution, that can fit on my 1024p desktop size, as a window.
And on my TV/monitor if needed.
I am trying out different amount of Graphic memory, this is the most amount of memory i have ever used
-using gpu_mem=256 seems to work fine too
Default framebuffer depth is 16 bit, which i use
Change to ex. 24 if you want more colors.