Much depends on the particular variety, the type of rootstock, and even your soil type. Having said that, I think 12 feet will be plenty of space; just be sure to put the grapefruit on one end in case it gets bigger. Anyway the grapefruit is probably the most tolerant of cold.

Thank you for your input, Johnmerr! The lemon is an unknown variety grown from a cutting, the orange is on sour orange rootstock and I am not certain of the rootstock on the Rio Red grapefruit. So, generally speaking, the grapefruit tree would be the largest? If left pretty well unpruned, what would be the average height/width of each of these 3 citrus trees?
Molly

Tree spacing is always a hot topic -- as most of the data is based on the commercial growers need for tractor access.

I laid out my citrus hedgerows on 8' centers. You can control the size of any tree by pruning and by not going crazy with fertilizer and water. If you have a large open area you can plant 3-4 trees in one hole as close as 18" to 2' on center. I've done this with peach trees, but not with citrus.

Here is a link that might be useful: The Art of Successive Ripening

I agree with John, and my orchard is planted much like mrclint's. Mrclint and I are on some pricey property in California, so every square foot is precious :-) I intensively plant because I can (nearly all trees are on semi-dwarfing rootstock), and also because I'll prune as necessary to keep size down. Plus, we don't have humidity issues like the gulf citrus states do, so maximum air flow isn't as much a concern for us in drier S. California. And, my stone fruits are planted like mrclint's, using Dave Wilson Nursery's "single hole" planting concept. Mine are in two tight rows that follow along a walkway. Not sure citrus could be that close and do as well as my stone fruits, but my citrus trees are probably between 8 and 12 feet apart. Just prune to keep the canopy open (down the road), so you get good airflow through the canopy as your trees mature. That will help to prevent fungal infections from having an opportunity to take advantage of a crowded situation.

Patty S.