The "A" dimension in the Aloris catalog for the tool holders (not the tool posts!) is essentially the lowest-off-the-compound spot the 'largest' tool's cutting edge would sit. So for normal tools, it is the ' top' of the tool-bit, plus the height of the area under the tool bit. For the boring-bar holder, it is the centerline of the boring bar (since that is where teh cutter is going to sit!), plus the area under the boring bar.
The "A" dimension on the tool-posts themselves is just the width of the tool post (which might be where the 2-1/2" comes from?). You don't want this to be too much larger than your compound dimensions (you want the post mostly supported by the compound), but otherwise, I don't know how relevant it is. "D" on that might be relevant, in that if the T slot in your compound isn't large enough, you won't be able to put the correct sized bolt in the center of it, and would have to sleeve it.
"C" might also be relevant, as you don't want your holders to sit too high and not be fully supported, so that would help you avoid getting 'too small' of a set.
ALL THAT SAID: I agree with Ultradog: Larger tool posts are better for rigidity. The tool holders themselves end up being way more expensive (for example, my CA tool post was a shock to me, where even teh cheap import holders are $35/ea, whereas BXA is 1/2 that!).
ONE THING TO CONSIDER: I wish I'd thought more about the Multifix tool posts:
http://www.createtool.com/list.asp?cid=33
They are a bit more expensive, but they seem to be REALLY useful/flexible. You won't be able to easily make your own holders with it, but the additional utility seems nice. BUT, by the time I discovered them, I was already nearly $1000 into my CA setup!
