spring onions

when they look big enough to eat 

You do mean spring onions as in salad onions?

What you can see is what you get. Some of them make a small bulb on the surface, some stay straight and look more like a small leek, it all depends on variety

how can i tell when my spring onions are ready  . . . . . . . . . . shall i just pull one out and see what it looks like. . . . .

Ravenzero,
I've combined your 2 questions, above.

You can see when a spring onion is ready. You don't have to pull one up to see what it looks like. 

I sow spring onions every couple of weeks in and around everything else and a few in tubs, bakers trays and pots.
Some I pull very young and thin; others I allow to grow on to fatten or produce large bulbs.
Just grab them when you want them any time after they begin to look like spring onions.