Your onions won't be scallion sized at transplant time but that's fine, they don't have to be.
That said, your onions at 5 weeks do seem to be a bit on the small size. My first thought by looking at the pic is that the soil looks dry. Onions don't like their soil to dry out and unlike most other plants, they won't show it by wilting.
I'd think making sure they are watered regularly and maybe give them some fish emulsion or your choice of mild fertilizer will help them pick up the pace.

I can get you a pic of my 4 week seedlings tomorrow if you want.

-Mark

Mark,
Yes please post a pic for comparison. They do look dry in the pic. I took before i watered. I do make sure they stay moist but don't want to over water either. I have not fertilized yet though. How often should I be doing that?

If you use fish emulsion, follow mixing instructions on the bottle. If you think they need it, you can feed every other week or more. There is no "right" way to fertilize as all potting soils have different amounts of nutrients. Watch for any yellowing as a sign that they need it and keep the moisture level constant, but not overly wet.

I don't suggest it, but If you choose to use some chemical based fertilizer, be careful to follow recommended amounts or even less. It's easy to burn plants with that stuff.

Below is a pic of my seedlings which were planted Feb. 1st. Sorry, it's not the easiest to see.
The don't seem much bigger than yours, but look at the second leaf on each plant and try to use that as a guide.