You can estimate the size of a plant's root mass by looking at the size of its leaves. Plants generally strike a balance: if the leaves form a rough sphere with an eight inch diameter, you can expect the roots beneath the surface to form a comparable rough sphere. There are no substitutes for familiarity and experience, but rules of thumb do help. Transplanting beneath clouds after a rain ensures two things: moist soil that will hold its shape when dug AND less stress in the form of evaporation and transpiration put onto the plant that has been dug. To dig a plant, picture the root mass beneath the surface, and then use your tool, here a trowel, to scoop the plant, roots and all, onto the tool. LEAVE THE PLANT ON THE TOOL. DO NOT PUT IT INTO YOUR HANDS. (For small seedlings you can use a spoon; you can use the blade of a knife for really small ones.) BEFORE YOU TRANSPLANT, be sure to ready the plant's new hole. Naturally, the new hole should be big enough to accommodate the root ball.
Plunk the transplant into its new location. It is better to plant it too low than too high. If you plant it too low, you can lift it up from beneath. Lifting a transplant from beneath actually helps any loose root tips to straighten out in the soil. Conversely, if the plant is too high, you may be tempted to tamp it down. Tamping a transplant is sure to stress it out more than lifting one, and minimizing stress is the name of the game when transplanting. Pampering is also the name of the game. Even though we are due for more rain tonight, tomorrow, and so forth, I watered in my collards after moving them. I will probably water them in again this afternoon. If you are not prepared to dote on a plant, don't transplant it. In this sense, plants are not unlike cats or humans. Moves are stressful.
Here, the flopped over plants that are showing the undersides of their leaves are the transplants. With proper care, they will right themselves soon. To help these transplants along (and to amuse myself), I made some splints for them. Many would say that this is a totally unnecessary step, that the collards don't need temporary splints, and those people may be right, but I would offer that one who makes a splint for a collard transplant is one who will later eat collards. The splint is indicative of the amount of required care. So care.