no...not any less likely to be sick in a 24 or even in a 34... but being crowded is not fun and may contribute to stress or anxiety due to some slight amount of claustrophobia, which might trigger seasickness.
but for vessel size, you would actually be much less likely to get sick riding on an inner tube.... or a vessel large enough that negates any swell or wave action.
with the inner tube, you are more an actual part of the wave action, and with a large vessel, there is no noticable wave action.... but everything in between is going to toss you around and make you fight for your right to be there. and until you win the battle, its going to beat the hell out of you and make you wish you werent in that fight.
but the good news is, if you can spend a few nights on it at the dock, or anchor out in a quiet cove, and in the daytime motor or sail out out for an hour or two and then return before you get to feeling sick, you will be able to slowly get used to the motion of the boat and you will build up a tolerance to it..... or, you can take it head on and spend a week on the boat sailing and see if your tough enough to take it. within 3-4 days of constant motion and seasickness, you should start getting over it and then you will be fine.
to much all at once can push many people past their tolerance level for motion sickness, but being able to get it in small doses will get them past it in time.... but then next season it could be the same all over again.
and there are also drugs that can be taken, but they sometimes have effects of their own that you have to deal with.