I used to be into wifi crap a lot.. netstumbling/etc.

The size of antenna doesn't have much to do with the length of your
cable (unless I'm missing something?) - except you want a pretty
powerful antenna since you're gonna lose a LOT of signal strength with
20ft of cable.

Check with these guys - http://www.fab-corp.com/ I've bought parts
from them many times. They are cheap AND good!

You'll want a different cable than the one you've picked out since that
cable doesnt have ends that will connect directly to your card. You
can get adaptors/pigtails to hook into that cable, but every connection
is a signal loss point. so you'd be best to get one piece of cable
that can go directly from the card to the antenna. (you CAN get signal
amps for longer cable runs - 20 ft isn't really that bad, but it's not
great - but they are expensive and still are not perfect since they
also amplfy the signal distortions)

Figure out what you want to do with your antenna and decide whether you
want an omni directional antenna with a nice ground plane (not those
cheap magmount ones), a patch panel, a parabolic, or maybe a yagi (if
you're gonna be pretty stationary, a yagi is probably cheapest - you
can make them from 2-3 bucks in parts and an empty pringles can and
they DO work pretty good)

Personally, I'd try to stick with a better quality cable, and try to
keep the length as short as possible. Either remount closer, consider
using an access point mounted by the antenna and running cat5 to your
laptop, or maybe consider switching to a usb antenna and running the
usb cable to the max length to give you less signal loss. Running
wireless over water can cause some serious signal problems, so you
wanna keep your signal loss to a minimum.