Assuming that your plants are healthy and free of root diseases, adding some nitrogen source will probably get your size up to par.

Yeah that nitrogen thing sounds like the problem. When are you planting them? Check you soil temps. They don't like to germinate when the soil is to hot but if it's too cool they grow slow. Once they get some size to them they come back fast after cutting. Are you growing savoyed or smooth leaf?

N is not always the answer to stunted growth. If the leaves are dark green, then N is not the problem. Not all spinaches are not the same, check the characteristics of the variety you have been growing. Also, try an early spring planting.

There have been lots of posts about this topic. I always do spinach starts (will plant soon) and transplant to the garden in late March, depending on wetness. With transplants you can be sure to leave plenty of room for each plant (4"-6" from the next) and you don't have to thin (hard on the back and hard to really pull out all those extras and sometimes nothing will sprout in an area)
I remember some have said the starts will bolt and not grow but I have never had that problem... they do bolt if we have a hot spell in late spring.

I learned a few years ago that I have to lime the area where I plant the spinach that year. Since I've been doing that I only have to deal with the leaf miners and the hot weather and the newly arrived woodchuck(s?) and rabbits.

spinach is affected by low pH too, and is also fussy about watering. I would try some lime or wood ash. To save time, you could try one row only urea, one only wood ash, one both.
water often.

Bella,
I usually pre-sprout the spinach seeds in damp paper towel, inside a ziplock bag. Once the tiny sprout appears on the seed, I plant them in 4packs of Pro-mix.
I often put the ziplock in the fridge for a couple of days to start, take out the seeds that have sprouted and return the bag to the fridge and repeat. A little crazy but I sometimes use a jeweler's loup and very big tweezers for this job(my eyesight is not what it used to be) It seems like a lot of work but I get very spotty germination when direct seeding in the garden and we often get hot springs lately. This way I can get the spinach in very early. I do the same with bok choy and other early greens like escarole.

I've had by far the best results by planting in mid-august, over-wintering the plants, and giving them a shot of nitrogen in the early spring, say mid-March.

The plants send down a tap root, and that keeps on growing until the ground freezes. The plants stay green under the snow, but will brown out if it's too dry on the surface. But the plants will shoot new leaves as soon as there is enough moisture and sunlight. Harvest is early - mid June, until it starts getting hot, or I need the bed for something else.

We have alkaline soils, which may well be a factor. We also have deer, who do indeed like their vegetables, particularly fresh green ones early in the spring. But by mid-March, night time temps will rarely get below 25ºF, its low-mid 50's during the day, lots of sun here, and the plants grow surprisingly well.

I've tried this with Bloomsdale Long Standing as well as Giant de Veuvrey (sp)

Check out the Winter Sowing forum. This might be the ticket for you. I had no luck last Fall, but WS'd two containers of spinach today. Folks on the forum say they have good luck with spinach and WSing.

I have this problem too. The spinach I direct sowed in my raised bed on Aug 1st are still tiny. The nice thing is they appear to be still alive, at least as of Jan. 1st. So I'm hoping I'll be able to do what david52 suggests above, and harvest them in the spring.