We too grow onions from sets and end up with large and small ones, this is perfectly normal and should be expected. When folk bought their onions in the greengrocers they were all different sizes, even in supermarkets the nets of onions are not all the same size. Think of onions like human beings - we come in all shapes and sizes.

You get uniformity by selection.
It is the same when growing from seeds.
I don't grow from sets any more, but when I did I always started them off in cell trays. If you want to grow 50 onions, set off 70 or more in trays. You will then select your best 50 depending on how they grow.
Same with seeds. Even though the seedlings are of a particular variety, you always find some develop better than others.
If you plant out good strong plants, you'll get good onions.

Hya Thank you both very much for your helpful advice, I feel a lot better now knowing there a few more of us out there just the same, I shall gert in touch again when my next gardening problem comes round, cheers

Hya Thank you both very much for your helpful advice, I feel a lot better now knowing there are a few more of us out there, I shall get in touch again when my next gardening problem comes round, cheers