You are gonna want 3" for the intake, the vr6 maf is 3", it makes no sense to go smaller, go bigger if anything. Turn2 was 3" tube and then flared out to run a 3.5" inlet filter, iirc. I was gonna go the ebay route for one since no turn2 or carbonios were popping up for sale but then some gave me a gift card to advance auto parts and I was able to make one for less that that.
For less than $50 I had a 3" cone filter, a 3" silicone coupler with clamps and a coated aluminum intake tube meant for a civic. Mock the tube up to see how much you need to trim off the long straight section. Then either remove the carbon canister or relocate it. I chose to delete it since it was cracked anyway. In a mk3 you may not have to cut the tube, I have a mk2 so I don't know.
I had trouble getting the filter I bought to fit between the fender and the edge of the front subframe but I have a mk2, I would say thats a problem but since its such a tight fit that I didn't even need to make a bracket to secure the intake tube from rattling/moving around.
In a mk3 I could have fit a much bigger filter if I wanted to, I should have measured the clearance for the filter on my car but I wanted a big filter so I just kinda squished the filter a bit and wedged it up there. My intake tube had a whole with a plug in it for some sort of pcv hose that hondas use. I used that spot to put my 1.8T IAT sensor into. Its right at the frame rail and reads nice cold air. All in all well worth the money.