I need this kind of stuff, but don't even know what thread size to get!
I'm thinking it depends on where you put it in your system, if you have a large compressor and want to install the combo close to it to filter and lubricate the entire system you would likely need at least 1/2" or larger.
I bought the 1/4 NPT and plan to put it at the end of my hard line and run my 3/8" hose with 1/4 NPT ends directly from it to my air tool I'm using. This will also keep me from getting my entire system contaminated with oil for when I want to paint, etc.
If I'm interpreting my specs correctly my IR 2135TiMAX uses 20 cfm at load, and the 1/4 NPT FRL combo has a flow rate of around 58 cfm at around 100 psi, which should be enough. The IR 2135TiMAX manual calls for a 1/2 NPT FRL combo though which would then neck down to a 3/8" line with 1/4 NPT ends into the impact, not sure if it actually requires this or if they just spec it to be on the safe side (if anyone has thoughts on this please let me know).
Either way for $15 I figured I couldn't go wrong. Even if it doesn't work for my impact and angle grinder it will at least flow enough for my nail guns.
For those of you who benefited from this, glad I could help.