Why 7x7x7 you ask? I simply ran out of money to buy the extra 169 leds and 2 extra TLC's to upgrade to 8x8x8. Good reason, huh?
I watched a ton of videos of led cubes before I settled on a 5x5x5 blue led cube. I think that cubes with odd numbers give more 'artistic' options for pattern development as there is a true center line for every surface. Just subjective, but that's my thoughts. And yes, even 5x5x5 was a challenge to solder up and keep alignment straight and lots or interfacing wires needed from the cube to electronic parts.
Lefty