how large should the robot be so the robot doesn’t get in the way ?
Just don’t be over 18" on any length of the robot at the beginning of the match.
Unlimited expansion is permitted.
Also, it would probably be wise to make it smaller than 18" because you might want room to add you launcher/lifter onto your base
I don’t believe you understand what they are saying. They are just talking about the size when the match starts and during the match
Skills competition setup this year has more elements on your side of the field. You may have to deal with them before expanding. Even in normal play the amount of clutter on the field preventing you from moving effectively could be larger than you think (especially if you are losing). All four cubes can be troublesome to maneuver around.
The length of an arm/catapult should be wisely made as well. So bigger may be better but not always practical. Waaay back in sack attack, teams had plenty of initially designed arms get shorter over time. High strength shafts and 12 motors should help a little.
Also, remember, you have another robot on your side (except in Vex U). You want to leave plenty of room for them to maneuver around you.