There are some tunes you can't play without wrapping your thumb over - Gary Davis sometimes covered the three bass strings with his thumb, and there are an awful lot of ragtime things where you need a G chord which is just the thumb on the third fret of the bass E string and the index finger on the third fret of the treble E string - you ignore the A string and use the other two fingers to access up to the sixth fret. The nut size is not the critical factor, the string spacing is critical. You can often have the spacing changed a little, but you have to be careful - the thing to avoid is having the outer strings so close to the edge that they fall off during playing. I've known a lot of people who prefer a fat neck, and people with big fingers who prefer a wide neck, but once you've found what you like the general rule is that you should play on the narrowest neck you are comfortable with - this may mean having the spacing widened on a narrow neck, but you'll play better for it. Most factory guitars have the strings spaced closer than necessary to avoid having to work to close tolerances (ie avoid the strings being too close to the edge of the neck), and there's often an eighth of an inch of extra spacing available.