What size do I need for my working?

For a full sachet blend with multiple herbs, several stones, and a folded petition paper, 4 by 6 inches gives you working room. It also fits a standard tarot deck at 2.75 by 4.75 inches without crowding, which makes it useful for card storage between readings, though it is not archival-grade protection.

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

For the ingredient prep phase, yes; that's what these are for. For a finished charm bag, cloth is the usual choice, most often flannel in a color tied to the intention, sewn shut with the contents inside. Cloth breathes and sits softly against the body, which plastic doesn't. Use the zip-top to organize and transport the components; use flannel for the charm bag.

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 seal isn't perfectly airtight against moisture, so herbs stored in humid conditions may still absorb ambient moisture over months.