I agree with crittahitta that the 98 cu-ft (or 100 cu-ft) is the way to go. There really is a marginal difference. The Omega 75 cu-ft'er is 6.3" in diameter, 24" in length, and weighs 10#. The 100 cu-ft'er is the same length, only 0.7" more in diameter, and weighs less than two pounds more. The only way you will know what is right for you is to actually try it. But I would not go under 75 cu-ft; it kinda defeats the purpose.
I used to have an Omega 75 cu-ft 4,500 psi CF tank and an Omega 4,500 psi Air Charger when I had my RAW HMx .357 (130 ft#). Since I sold my RAW and switched to the FX Wildcat .25 Mk1 (similar to your situation), I sold my tank and compressor. I thought I could handle the relatively small 300 cc bottle and 230 bar bottle fill pressure of the Wildcat with my FX 4-stage hand-pump. Well, after weeks of filling my bottle with my pump, I finally broke down and ordered an Omega 100 cu-ft 4,500 psi CF tank from Airguns of Arizona. A 100 cu-ft'er means less trips to the dive shop (not my favorite activity). I always have issues with waiting overnight and my tank not being topped-off. (This is the reason I bought a compressor.) But, now that I have only my Wildcat and I am strictly a hunter, a 100 cu-ft'er w/o a compressor should do me just fine. After all, I live in San Diego; there's a dive shop on every block. We'll see. Like most things in life, you never really know how something will be until you actually do it.
Keep in mind that tanks must be hydro-tested every five years or dive shops will not fill them. So be aware of this when buying used.
