An "even number" is an integer that is divisible by 2 so can be written in the form 2k for some integer k. An "odd number" is an integer that is NOT divisible by 2 so can be written in the form 2k+ 1 for some integer k. You don't say what kind of thing has "even size" or "odd size". I presume it is something where "size" **must** be integer. In that case it has "even size" if and only if its "size" is an even integer, "odd size" if and only if its "size" is an odd integer.

(That's my guess at what you are asking. You give your location as "Sydney" (I presume Sydney, Australia) so it is odd that your English is so hard to understand.)