What size blades do you use for 1/4 oz. and 3/8 oz. double willow spinnerbaits?

1/4 oz you can use a #4 and a #3 possibly a 3.5. 3/8 oz you can use a 4.5 and a 3.5, possibly a 4. I generally go 1 size down on the front blade, but have experimented with going to 1/2 sizes down. The biggest single blade on a 1/4 oz I use is a 4.5, but that is fishing real slow and I haven't tried a double willow using a 4.5 on a 1/4 oz head. 1/4 oz spinnerbaits can be real unstabe, so I would build one and field test it before you go nuts building a lot of them. 3/8 oz spinnerbait heads are a bit easier because they are not as unstable, IMHO.

Hope this helps.