What size omni wheels should we use on our robot for vex in the zone??
Most years, wheel size is about desired speed/torque tradeoffs. Since wheel size is, of course, related to the across-the-ground speed, like motor/gearing choices, it depends on your design goals.
This year there is the added issue that you probably want to be able to climb and cross an on-field obstacle (depending on your robot design; goes without saying, but I said it.) Larger wheels make it easier to climb.
Testing your choices is the best way to determine it. I haven't personally seen anyone use the 2.75 omni wheels this year. I've seen both 4 inch and 3.25 in testing. I liked the performance of the 4 inch wheels better, but I'm not building a robot. Well, not a robot for this competition.
@Nathan004 re using omni's on our bot this year, we decided to do dualies on the back to get over objects, so far it has proven to work
generally, most teams use omnis as the standard choice, as they work well and defense isnt really a huge deal in the US