Size? Well, it has to fit in the space you have in the garden, and it has to be large enough for the plants you want to grow.

Position? Do you want to place it directly on the ground for planting into the soil? Or put it on a base (slabs or concrete) for seed trays, growbags or pots.

Height? Traditional frames are low, being designed for raising seeds and cuttings. The taller mini-greenhouse types are useful for protecting plants such as tomatoes or peppers from seedlings through to harvesting, but may require a wall to form the back of the frame, and provide support. You could also fit racks in these tall types to take lots of seed trays.

Access? How are you going to put plants into the cold frame and tend them? Some cold frames have hinged lids to lift up but these can be caught be the wind, lifted, twisted and dropped. And we found those traditional wooden framed lights to be very heavy and awkward to handle alone - drop them (we have) and inevitably you'll break the glass (we have). Sliding panels or roll-up tops are much less damage prone.

Reach? If you site your cold frame against a building and block access to one or more sides, you need to check that you can reach the furthest points within. Otherwise you will have to stand in the frame to reach all the plants - not easy if plants are growing in the soil and a nuisance if you have to keep moving pots and trays around.