As DarthViper suggested you’ll need to go into the details panel on the right side once you’ve selected the face you want to adjust the texture size of. You’ll find an option to adjust the Scale properties. Adjust the U and V drop down to the size you’d like. Smaller will make your texture resolution increase and higher values will make the texture resolution descrease. Once you’ve selected your values hit apply and you’ll see the UV for that face change accordingly.