How long of a cable and what type do I need to extend my wireless signal?

There is no such value - every additional cable will get your signal weaker. General rule is that the shourter the cable the better. Thats why you for example use outside boxes which are tempeture resistant and you provide power via POE (power-over-ethernet).

I understand.  Very good point.  Would you say I should be fine if I'm using a 50' cable and sending the signal to a building 150' away?  I will need to receive the signal from within the building.  Its a vinyl siding exterior on one and brick on the other that is much closer.

In practise you can use cat 5 cables up to 300 feet long without signal problems (mostly related to latency problems rather than signal strength I think). If you can't run the 150' cable (in conduit) between the 2 buildings, then you can mount a directional antenna on the outside of each building, and connect them to the antenna ports of 2 wireless bridges, to link the 2 buildings. Or you can have one attached to a wireless router and in the other building have a wireless repeater which will boost the signal strength it receives, and re-broadcast it inside the 2nd building. Depending on where you are, I have even installed cheap Linksys wireless devices in tupperware containers under the sheltered eaves of buildings - no need for specialized antenna connectors or co-ax antenna cable.

Let me clarify, the cable i'm speaking of is a cable attaching the antenna to the access point.

Even more info  on cabling if you want it:
http://www.l-com.com/multimedia/catalog_page/2K9M1_124-163%20Coaxial.pdf