Check out the
pinned topic about mesh sizes - very useful.
It really depends on a load of different factors as to which screen size to use, but mostly the size of the trich heads on your particular strain (e.g. sativa heads are smaller than indica heads apparently - check bubblebag threads for that kind of info).
I have used a variety of silkscreens - 43T (150u), 53T (125u) and 77T (75u) to try to achieve full melt dry sift and not had very good results. So much so that I ended up saving up for a proper tumbler. I have seen loads of folks getting great results using 43T, so I'm pretty certain it was me screwing up or doing something wrong....however.....after reading Bubbleman's ic thread about his sift making method using the box (or screens) I couldn't make it work using my set of screens (but his are 160u, 140u, 107u and 70u) to further refine my sift.
If you google "woven wire mesh micron UK" you'll find places that sell stuff. It's around £30/m for the material alone if you want really proper stuff, otherwise google away for silk screen materials but you'll have much more limited aperture sizes (micron hole size).
At the moment I get the best results using dry frozen trim and tumbled in a 120lpi drum for 5-15 mins first run. I get full melt bubble finally. If you're on the same kind of mission as me I can save you time and money and suggest a good tumbler and drum.
GN
edited to add link