Well if its for your entire computer screen, and its a windows PC, right click and go to properties, go to settings, and then there will be a bar where you can slide it back and forth.
800x600 is too small, all icons and stuff are like too big, you cant open your inventory without it blocking 2/3 of your screen, while on 1280x you can open like a lot windows before it really blocks your screen.
well just choose a option
i gave u three thinks too choose:
The first is good if u want to change it more then 1 time.
The second is the savest one
The third, is when u want to know what i did.