Of course you can.

Think in terms of cups of measured leek and don't worry if they are big or small.

You may want to substitute scallions or spring onions. I've used them with success...

My leek supplies are pretty irregular, so I do tend to use whatever I can find. Even when I use them raw (in potato salad...), either works fine. If you're cooking them it should make even less difference.

I would just look for the slenderest leeks I could find.