Sara! Good to see you here!
I don't worry too much about thread size, unless you're mixing thread sizes in a single piece or trying to make something to fit, it doesn't matter, the whole work will come out to scale with the thread used. When working with a new thread, I work up a little motif and compare it to other threads I have the same motif worked up for, to get an idea of gauge. I've also had good success with measuring those motifs and doing up the math to re-size a work as needed.
I didn't post all the boring math, but I used that technique here: http://www.intatters.com/showthread....ridal-Necklace
The math was basic algebra, if thread A produces a motif x times as wide as thread B, then the entire work made from thread A will be x times as wide as the one made from thread B. Apply to individual elements and figure out which of them to duplicate or remove to get the final size you want...