if i have a 2002 X, and I wanna slap on a flowmaster 40, what sizes should i keep in mind when buying?

What exactly do you mean? As far as I understand the muffler is the same size but you might want to go with custom piping that's 2.5 in diameter as opposed to 2.25 stock size. That's about the only number to keep in mind. Flowmaster 40 is good. I have the 40 series on my truck with 2.5 inch piping and it sounds great.

When you first get it on it won't sound all that good. you have to wait a little until there's some carbon residue on the exhaust, then it'll sound much nicer and deeper.

2.25 is fine if mandrel bent otherwise 2.5” stock is actually 2 from the muffler back.

Consider the 50 series SUV line; I paid $80 for mine at truckperformance.com. Larger and quieter than the 40 series with a nice mellow deep tone.

I wouldn't say that the 40 series is that loud - not at all.

I like the sound of my 40 series. I picked it up brand new in the box off E-Bay for $40 plus shipping.
Cost me $70 to have it installed with 2.25" piping and bent to look stock.