What size do I need for a sachet or spell bag?

For most sachet and small-spell work, 3 by 4 inches is the practical middle. It holds a tablespoon or two of dried herbs, a small stone or curio, and a folded petition paper without crowding. If you're building a full mojo hand for wearing on the body, the finished cloth bag itself is usually 2 by 3 or 3 by 4 inches sewn shut, and the zip-top is where you keep the ingredients while you gather them.

Can I use a plastic zip-top bag instead of a traditional cloth mojo bag?

For the ingredient prep phase, yes; that's what these are for. For the finished working itself, hoodoo tradition calls for cloth, most often flannel in a color tied to the working's intention, sewn shut with the contents inside and fed with condition oil. The cloth breathes, holds the fixing oils, and carries the working on the body. Plastic doesn't do those things. Use the zip-top to organize and transport the components; use flannel for the mojo hand.

Do I need to cleanse the bag before use?

Not strictly, but many practitioners do, and there's no reason not to. Rinse the empty bag with cool water and let it air dry, or pass it briefly through cleansing smoke, before putting consecrated contents inside. The intention is a clean container rather than a purification of the plastic itself. If you're storing already-charged materials for a working that matters to you, treating the container as consecrated is good practice regardless of what it's made of.

Are these safe for storing herbs long-term?

For dried herbs in a cool, dark place, yes. LDPE polyethylene at 2 mil is standard light-duty poly, chemically inert, and holds dried plant material well without off-gassing. For long storage or damp materials that need more protection, step up to a thicker bag or a glass jar. The zip-top seal isn't perfectly airtight against moisture, so herbs stored in humid conditions may still absorb ambient moisture over months.