buying bigger frame to get less aggressive geometry If I like a model of bike (looking at giant TCR 2) but the sitting position of the bike is a little too aggressive for my liking i.e. too bent forward instead of upright, is buying a slightly bigger frame and lowering the saddle a good approach to lessen the crouching forward?

I'd consider that as a last resort rather than a first resort, since you can probably accomplish the same effect by getting a shorter stem and/or raising the stem. If you can't change your position enough that way, you might want to consider getting a bike with different geometry.

You can quantify how aggressive a bike's position based on its "stack:reach ratio" (K:L in the diagram below). On the TCR, that number is about 1.4, which is indeed pretty aggressive. Giant has a couple of other models that are a little more upright, the Defy and Contend, where that ratio is about 1.5, which is typical of "endurance bikes."

!geometry diagram