Is it like this?
if part.Size == Vector3.new(5,5,5) then
OK, so I just figured out that sometimes this works and sometimes don't, because the size of a part usually goes up and down very very very slightly sometimes, so how do you prevent it?
For example: A big part with the size 500,1,500 can sometimes become 500,1.00000000005,500 and yeah... that's really annoying
The same way you might compare two part positions. They likely won't be exactly the same so check an arbitrary distance.
if (part.Size-Vector3.new(5,5,5)).Magnitude < 0.1 then