What space and setup does Reishi (Ganoderma) cultivation need to start?

For a small Reishi trial you need much the same kind of facility as for shiitake, since both use sterilised hardwood sawdust blocks, so this is more demanding than an oyster room. At minimum you need a sterilisation unit such as an autoclave or large pressure steriliser to fully sterilise the supplemented sawdust blocks, a clean inoculation area such as a laminar flow hood or a good still air box, an incubation space to colonise the blocks over several weeks, and a fruiting room where you can hold warm temperatures with very high humidity and controlled air, plus a way to dry the harvested brackets. A small spare room can work for the trial scale, but the sterilisation and clean inoculation equipment is the real requirement, not floor area. Because the cycle is long and contamination risk is high, start with a small number of blocks. And I will repeat the honest caution, sort out a genuine verified buyer before you invest, because the market, not the space, is the hard part. Get the method and equipment list from DMR Solan or an SAU.