If the booth is completely dead then the size doesn't matter. But is it completely dead? Your ears are not going to tell you. All that foam in there will stop the mid and higher freqs but will have little affect on the mid-lows and low freqs. Your ears are attuned to the higher freqs, so it sounds dead, but the troublesome spots will be like 300hz - 700hz for a booth that size, so a vocal track can sound muddy.
Maybe you can EQ out the mud, but a good vocal take is one that sounds good at the mic, not the mixing board and the mic is going to hear something different than you are. If the booth doesn't sound good, then the vocal take isn't going to sound good. Common wisdom is to have a large room with a lively, "designed" sound to it.
But hey, we're home recording type people, we have to make do with what we got. Try it. If you like what you hear then you're good to go.