CFM (cubic feet per minute) is KING where important specifications go for OBA systems. To be a really useful compressor for refilling our Jeeps's big tires, you want something at least 2 CFM or higher. And be wary, many CFM ratings are totally misleading, like the CFM rating given by Smittybilt.
In order for CFM to be a valid useful rating, the air pressure that CFM was achieved into has to be included with the CFM rating. A CFM rating into 0 psi will be useless, as that 0 psi rating meant there was no resistance against the compressor. Your lungs can probably put out 10 CFM into the open air but 0 CFM into a tire. So you want a CFM rating that says it was made into some pressure, like 20 psi. That means it can put its 2 CFM rating into a tire that has 20 psi of pressure pushing back... a useful way of knowing just how powerful the compressor really is and if you'll need only 5 minutes to refill your tires or 45-60 minutes to refill your tires.
That Smittybilt I mentioned brags it can put out 5.65 CFM... which is HUGE. But that's not how much it can really put out in actual use when airing up a tire... their 5.65 CFM rating is into 0 psi, totally misleading. Given its size, I'd guess its honest CFM rating is maybe 2 CFM at the most.
My best advice is to buy the compressor with the highest CFM rating you can afford, just make sure that CFM rating is an honest one... look for what pressure it was rated again. ARB and Viair compressors are known good quality compressors. I'd avoid anything sold in an auto parts store, Smittybilt, etc. Ignore inexpensive compressors whose biggest claim is an extremely high PSI rating... even a bike hand pump can achieve high psi into a small bike tire, I had a hand pump for my road racing bike that could pump a tire up to 120 psi relatively quickly... but that tire was extremely skinny and didn't hold much air. That same bike hand pump that could achieve 120 psi in my road bike tires would probably take 2 hours to pump up my 35" tires to 25 psi.
On my previous TJ (stolen), I converted a York belt-driven a/c compressor which put out a LOT of air... probably 7 CFM if I raised the engine up just off idle. I miss that OBA system, it even drove air tools very easily. For my replacement TJ, I put together a CO2 system using a 10 lb. aluminum tank. It works well, it puts out around 5 cfm, I just have to remember to refill it every 5 trips or so.
Hope that helped.