If you "have them" in cell array form, then you could do something like this:
>> vectors={[3 4 5],[1,2],[5 6 7 8]}
vectors =
1×3 cell array
{1×3 double} {1×2 double} {1×4 double}
>> [~,imin]=min(cellfun('length',vectors));
>> smallest=vectors{imin}
smallest =
1 2
