MAFS size does not dictate HP. You pick one that will read more than the amount of hp (volume/mass of air) that your setup will make. If you're making 300whp, then pick one that will max out higher than that.

Size is only loosely connected to hp rating. An 80mm Z32 MAFS will read up to 500whp. But the N60 Maxima MAFS is the exact same size and only reads to 350whp. S13SR and S14KA have the same diameter but the S13SR one gets really narrow in the middle... the S13SR supports about 70hp more.

If you need a bigger mafs than a z32, you're looking at a Cobra mafs or some other late-model american car. It's also possible to get mafs elements put into different sized (larger) tubes and then flowed.