Time Trial / Time Trialling / TT

are there any good guides around for building a TT bike on the cheap? I already have some bits kicking about and I don't need to buy wheels, so I'm considering cobbling something together slowly over the winter.
What is the deal with frame sizing - go a bit smaller/shorter than you would for road so you get the right elbow bend and a low position?

I'm hopefully taking inspiration from this thread over the winter. Again, just struggling to work out what frame dimensions would best match my current (geared) TT bike.