I currently in the process of converting 1000 sq ft of attached garage into living space while doing this we are updating electrical service to 200 amp new service, there is 100 amp service in the old part of the house which will remain but need to be sub paneled from the new 200 amp service. I have read the forms on sub panels which most discuss needing to use a 4 wire with in the same building but not sure what size and am confused? Just what size wire do I need to use?

To feed a 100 amp subpanel, you can use #2 aluminum 4 wire SER cable, if aluminum feeder cable is allowed in your area. Otherwise you can use #4 copper wire, in metallic cable called MC cable.

It does depend on length of run... 1000sqft garage is huge. Lucky you.
I agree with tk, although I'd be inclined to use ACWU... not that expensive and the armored aspect gives you more flexibility (SER needs mechanical protection, like walls etc). Noticed this thread is ancient, but I'll just leave this as an option.

What is ACWU?

http://www.southwire.ca/en/products/acwu.htm

Ahh... good stuff!

Note the original post was 2007. Probably wired by now!

Yeah, he knew..

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I agree with tk, although I'd be inclined to use ACWU...

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Are you in Canada? We do not use this cable very much in the US if at all, and good luck finding it even in a supply house.
SER only needs physical protection where subject to damage. Not every exposed installation is exposed to physical damage.
I think that's what you call Tek cable? Does that give any more protection value than SER/NM? In the US MC cable has no more protection value than SER/NM.

Edit: Yeah, I see the link is a .ca domain, so your suggestion applies mostly to Canada as opposed to the US. There are some differences so it would be good to note this next time you reply to a 7 year old thread. :)