Let me get this right you are talking about when you first bootup. You are looking at the Gigabyte Logo screen right. It will never change because there is no driver installed in Dos mode. I don't know about the AsRock board.
I have been doing this for 35 yrs and I have never seen or heard that you can change the resolution of the boot logo. If you can then there is something I learn today.
Uh.... what I believe the OP is saying is that the video card (GPU) is not displaying in full resolution in the UEFI BIOS. This usually happens when the UEFI BIOS doesn't properly the video card UEFI. Anyway yes you can (in a sense) toggle the resolution in a UEFI BIOS. However, as I said, if the UEFI BIOS is displayed in a low res state, the GPU isn't properly being read as UEFI capable. However, this wouldn't stop the card from fully functioning in Windows, it's just a UEFI BIOS thing. That said, it CAN affect your ability to use secure boot.
BTW I'm having the same issue with my GA-Z170X-Gaming GT board, where my Sapphire Tri-X R9 Fury is doing the same... not running in full res in the UEFI BIOS. However, this unfortunately is a known issue with some of these cards...
https://www.sapphireforum.com/showthread.php?34123-R9-Fury-Tri-X-won-t-work-with-CSM-disabled In this state, the card will also not display the UEFI BIOS in full res. It has also prevented me from enabling secure boot. Fortunately though the card works fine in Windows, so no issues there.
At the OP, it may simply be that the card is not UEFI compatible, and/or you have CSM enabled. Try
disabling CSM, and see what happens. Warning!!!... if you disable CSM, and the video card does NOT allow the system to output a video signal... you'll need to clear your CMOS. If you're on the
Intel graphics, disabling CSM won't kill the graphics (not output a signal), so you would be good there, and you should get a full res UEFI environment. Been there, done that.
Good luck.