Decoys can be any size. A few, smaller decoys would give you more flexibility if want to lessen a strike rather than commit everything.
I think the confusing part is that there appears to be no reason, mechanically, to make decoy missiles that match the ship size, or in fact anything larger than the minimum decoy size. As far as I can tell, only the total mass/signature of decoys determines the fraction of missiles deflected, and cost scales linearly for both decoys and launchers.
Intuitively, I think players expect a decoy to "look like" the ship it's decoying from, i.e., to have the same signature. I can see why the actual mechanic is different, as its more flexible and less micromanage-y than requiring exact size matching (plus avoid issues with ship sizes that aren't nice, round numbers), but the fact that an infinite swarm of size-5 decoys is arguably optimal (same performance and cost as any other option, maximum flexibility) seems to eliminate what could/should be a gameplay decision point.