When I started my journey with Fedora, the low resolution in GRUB bothered me terribly. Now when I don’t use dual boot, I don’t care. 
Fedora sets by default
GRUB_TERMINAL_OUTPUT="console"
in /etc/default/grub
Change it to GRUB_TERMINAL_OUTPUT="gfxterm"
Update GRUB. UEFI system:
grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
Non UEFI:
grub2-mkconfig -o /boot/grub2/grub.cfg
That should help.