If the gene is smaller, is the protein that it codes for smaller too?
I guess that is true, and even accurately true, with certain caveats.
One would be that by 'gene' here you'd have to mean coding sequence or the total length of exons. That is in eukaryotes the DNA coding sequence that represents the sequence of the protein product) can be interrupted by nonencoding sequences called introns so if you counted these the relation would be a lot more variable.
Another caveat would be that by protein you have to mean polypeptide. Functional proteins on more often than not are assemblages of for example two, four or six etc. Identical or nonidentical polypeptides.
Then sometimes there is post-translational processing - cleavage by peptidases of the initial polypeptide product.
And no doubt other complications.
But except for these overlaying factors, in the simplest cases the answer to your question is yes.
Great, thanks epenguin, yes, I was wondering whether in general cases it checks out to be related!
Separate names with a comma.