I found the great thread about fixing door seal leaks by removing the door panel and replacing the seal. It mentions using POLYshim Butyl Architectural Tape . Does anyone know what size to get and is there an easier to find replacement? caulk?

Caulk is not a good replacement whatsoever (don't ask me why I know). Go with the butyl strip and get it done right, and have the ability to remove the water shields again and again. Any size butyl should work.

Go to any auto body supply store, and ask for the stuff used to install older windshields, or, order it off Ebay. Guns is correct in that is it butyl, but you want a bead, not a strip. The stuff shown in the following link is exactly what you need:

Auto Glass Sealant Adhesive Butyl Tape 15' Roll Soft Seal 1 4" Round | eBay

1/4" round is the size you want. 3/8" round is okay if that is all you can get. Seems to me a 10' roll does one door with some left over. I am not sure if a 15' roll will do 2 doors, but it would be close. Don't be afraid to piece together left over pieces ... this stuff sticks to itself (and anything else) very well.

Crap. The OP posted this a month ago. Hope he sees it.