I would recommend 2.5" or bigger.

Everyone will tell you to run a 3" exhaust and a big ass Magnaflow muffler, but 2.5" seems to get the job done for an awful lot of people. The key is mandrel bends and smooth transitions.

Standard T25/T28 Turbine inlet:
https://www.miataturbo.net/attachmen...ine=1351205608https://www.miataturbo.net/attachmen...ine=1351205608

https://www.miataturbo.net/attachmen...1&d=1351206003

Standard T25/T28 Turbine outlet:
https://www.miataturbo.net/attachmen...ine=1351205608https://encrypted-tbn2.gstatic.com/i...OOLwuXoG3-VW-Q

https://www.miataturbo.net/attachmen...1&d=1351206003

I can't say I've ever seen a "circular one with 2 bolts" but if you are seeing T25s with a 4 bolt compressor inlet and a round hole, it may be a TD05 from a DSM. Check out Joe's thread: https://www.miataturbo.net/useful-sa...flanges-52542/ for more details

I did a full 2.5" on the last build I did before I sold the car and had great results. The typical T25 flange has 5 bolts. The 300zx T25 turbos used a 4 bolt flange. Go with the 5 bolt.
300zx t25 turbo:
https://www.miataturbo.net/attachmen...ine=1351220525

GT2554 is the stock turbocharger used on 2003 and 2003.5 Mazdaspeed Proteges. Check out Mazdas247.com, in the classifieds section.

Oh, and I'm new here so, Hi...

Three inch downpipe and exhaust with bigass Magnaflow muffler as stated above. Three inch will be more efficient and will give you headroom if you decide to upgrade later. It will work with a 2.5 but will work better with a 3.