How do you back/frame and hang posters of all size

It isn't rocket science. Use acid free double sided non-permanent tape, like the kind used by scrap book people, and mount the poster on acid free foam core.

As for matting, I use a Logan mat cutter to cut mat. To make frames, get a corner clamp. I don't cut my own glass, only because i use UV glass and it doesn't cost any more to get it cut to size.

I like to have things neatly framed but I'd rather do it myself because I'd rather be in the workshop than listening to someone else watching TV.

Any large online art supply house, like Jerry's Artarama or Dick Blick or Cheap Joe's, will sell metal channel framing in pairs. You pick two dimensions (l and w), order the pairs, and you'll be able to put together black gallery-style metal frames with nothing more than a screwdriver. The metal channels have corner brackets that fit inside the channel. Here's one example, but they're sold in many styles: LINK

Acid free tape and acid free foamboard to mount.

Hardware store for glass, though I seriously recommend Plexiglass if the image is very large. It is lighter and far safer.

Frame a few things yourself, and you'll never go back to the ridiculously overpriced Michael's or Hobby Lobby framing depts. Ridiculous markup!

And if someone wants a very low cost alternative custom frame job, I have also ordered custom frame, mats and glass from Frame Destination. You have to provide precise measurements but they will ship you all the pieces to put it together. I have used them twice for things when I didn't have the time to do it myself and both times it was a flawless transaction.