Sorry for such a noob question, I have search to no avail.

What is the minimum brush face size to be portable?

I would like to say 64 x 128 but you may have to experiment a little to get the exact size. Thats where the dev measure textures come in handy.

62x109 is the smallest brush I can get a portal on.

My suggestion is to find a portal prop, then reduce the size of the brush until it meets the edges of the portal prop.

I would still recommend 64x128 just so textures look nice and to keep things to the grid nicely. Also try to make portable surfaces at least 16u thick (I think you may be able to go down to 8 thick) otherwise portals occasionally flicker or have visual artifacts at longer distances.