The smallest gas forge I've built was out of old freon bottles. I use the plural as they'd eventually burn up, due to heating and cooling and being outside in the weather and rusting and the flux from forge welding eating the lining and the metal up over time. That was with a venturi style burner made from pipe fittings. When I went to a larger forge I found I got better efficiency from blown burner than venturi. For all around general work I would not go smaller than a freon bottle size. As an aside, I only ever used 1" thick ceramic wool, just coated with Satinite and ITC-100. Bought a roll of Ceramic Wool 15 years ago and have built several forges and still have wool left over for patching and relining.

If all your wanting size wise is to forge razor blades and smaller knives, why not a one or two brick forge? Basically all you need is a couple of soft fire bricks and a propane or map gas plumber's torch. The map gas works better. Take two brick and carve out a hollow in either side with a hole for the torch nozzle and coat the inside with light coat of Satinite and ITC-100, use a little Satinite to glue the two halves together and use tie wire to wire them together. Set up on a couple of normal hard fire brick and rig up a large coffee can to hold your plumber's torch and your good to go. With map gas you can even do small billets of damascus, though the flux will eventually eat the brick up. Use a large coffee can to hold the plumber's torch as you can put water in it to keep the bottle from freezing up, though I've never had that issue. Lots more efficient than a larger, normal forge. Or take a look at youtube and the Coffee Can forge, same idea, except instead of soft fire brick your using ceramic wool and a large metal soup or coffee can.