i have made a couple.
i use 1"by oak if i can find it. mainly cause i like how it looks once stained.

don't use pine. i broke pine on the ass of a certain partner . It was my first paddle made of laminated plywood. don't use that either. lol.

I like a board that is 6 to 8 inches.

I liked holes at one time but i've found you can take a router and make all sorts of ridges and other shapes which leave wonderful paddle marks.
the half round or the v shape bits are most excellent to me.

hrm oh and the length. when i was younger i made one that was nearly 4 foot long not counting the handle.
so i could spank two at once. i dated twins then....
still have that one.

but my last one is only 18" not counting handle. i think it is a nice length. not too long and very maneuverable in most places.

hope that is a starting help.