As far as I know onion size depends on variety, soil fertility and ample water. I mostly grow Maui (sweet granex) and some green/red onions/scallions, so I don't have experience with your climate or onion varieties.

Here are some of my onions as of this morning. I use a lot of compost and plant them pretty tight:

Thanks, mrclint. I know you'd be planting before we do, being much farther south and not needing to wait until snow thaws & evaportates/drains. Those are some nice onions at this April stage!

Anyway, how much are you irrigating your onion crop now, and how do you handle the irrigation in your mid- and late season?

These aren't great storage onions so I keep even moisture in the soil through out the season. If they get too dry they will bolt and quality suffers. I will probably harvest a few here and there and at the first sign of bolting, as they are so good caramelized in a skillet. I usually let them cure on the counter for a few days before cooking.

I'm in a very dry climate with negligible rainfall. I grow Candy onions and Music Pink garlic.

I've found the trick is to keep the soil 'moist' throughout, which I do with a heavy mulch. I usually have to irrigate once a week.

joel_bc, I should mention that I plant onion seedlings in the late Fall and grow them over-Winter. If all goes well I will be harvesting them into June.

mrclint, you wrote: "joel_bc, I should mention that I plant onion seedlings in the late Fall and grow them over-Winter. If all goes well I will be harvesting them into June."

I like the idea. I plant my garlic in October and November, and I mulch it with straw and it over-winters under 18" of snow. But garlic is pretty hardy. Possibly onions planted from sets would be just as hardy, I'm not sure. But I usually plant my onions from seedlings, and as you'd know they're far more delicate.

Unfortunately many gardeners have trouble growing those jumbo, softball-sized onions that most commercial growers produce on a routine basis. To grow them like the pros do, you need to select the right variety, plant at the right time for your area and maintain optimum moisture and fertility.
Onions bulb in response to day length and are classified according to how much daylight they require to bulb. The short day onions need 10 to 11 hours; intermediate varieties need 12 to 13 hours; and the long day varieties need 14 to 16 hours. The day length is just not long enough in South Texas for the intermediate and long day varieties to bulb. Short day onion varieties will produce large bulbs anywhere in the US plus they are milder than the other types. So, unless you garden in the states, where the intermediate and long day varieties will do just fine, I recommend that you stick with the short day varieties commonly referred to as Bermudas. Within that group is the white Bermuda, southern belle, white granex, yellow granex and the famous 1015Y Texas supersweet. Incidentally, “1015” stands for October 15, the optimum time commercial growers plant their onions in the Rio Grande valley. Next to variety selection, timing is the most critical aspect of growing large, succulent onions. In the spring, the best time to set out transplants is 4 to 6 weeks before the average frost-free date for your area. By planting then your plants will be at the right stage of growth to start bulbing when sufficient day length is reached. In cold areas, gardeners should avoid planting in the fall as severe cold weather can damage the plants or cause them to break dormancy and go to seed instead of producing a bulb. Bigger is definitely not better when it comes to onion transplants. Select onion plants that are less than the diameter of a pencil or you will have problems with them breaking dormancy and going to seed as mentioned above. For best results, obtain plants from a local grown sources, either your local nursery or a mail order grower. Avoid the bulbs and sets you see in catalogs as these are often the long day varieties and will surely disappoint you. You can also grow your own transplants from seed if you have a greenhouse or cold frame. Onions will grow well in well drained soil with a pH above 6.0. The addition of compost will benefit both light sandy soil and heavy clay soil. If your soil is on the acid side (below 6.0 pH) you will need to add lime to raise the pH to above 6.0. Onions do extremely well in raised beds that also make cultivation easier for the gardener, improve drainage and warm the soil in early spring. Onions really respond to the addition of well rotted barnyard manure. Apply at the rate of 1/2 pound per square foot prior to planting. Also, apply about 1/2 pound of a balanced organic fertilizer per 50 square feet to the bed before shaping and planting. Onions have a relatively shallow root system and fertilizer and moisture need to be applied in a rather narrow band around the plants.

One important thing to mention is that alliums cannot handle weed competition. Keep them well weeded!

I would go easy on irrigation. Last year was wet here (S.E. Michigan) and my (and other) onions were double-hearted and did not keep at all well because of the inner skin. Regards, Peter.