What size greenhouse should I buy for a backyard?

For seed starting and overwintering a few plants, a mini 2x4 ft shelf-style unit is enough. For growing tomatoes, peppers, and a real spring-to-fall crop, owner threads on r/Greenhouses and r/Allotment consistently recommend 6x8 ft as the practical minimum, a 6x4 is workable but cramped once shelving goes in. 8x12 or larger is the sweet spot if you plan to walk rows and overwinter potted plants.