For years my standard pocket notepaper has been 4x6 file cards cut in half to 3x4. It is a perfect size for a shirt pocket. You could also do it with 3x5 file cards if you want the lines going the other way, and I have a small stack of those for a specific application.

One hint that I learned from my wife: any time I write anything down on a clean sheet I date it (including the year). Cuts down staring at it six months later wondering whether the information is still relevant.

And, yes, I have a four inch high mostly unedited stack of 3x4 cards. I can find anything there, given six hours and a lot of luck.