I am guessing that you don't know what variety of blueberry shrubs that you have. If they are 7 feet tall, they must be 10 years old, or older. Berry size depends mostly on the variety. We have a half-high "friendship" variety blueberry shrub, and the berries are quite small, every year, consistently. The fruit on our "patriot" variety shrub is nearly twice the size. Its not absolutely necessary to fertilize blueberry shrubs. I give our shrubs a tablespoon of Miracle-Grow (acidic formula) every spring, when they first begin to leaf out. This year, I gave them a second application of fertilizer in June, because most of them have a heavy load of fruit. Commercial blueberry growers apply considerable fertilizer, much more than I do.

I agree with Ericwi, as the berry size is dependent on the variety, a shot of iron sulfate every now and then won't hurt.

Whether or not BB get fertilized is dependent upon what the soil is providing. All plants, including BB, need nutrients to grow properly. While there are many home growers who don't fertilize, as far as I know all commercial producers do.

If fertilizing (appropriately) didn't have a positive affect on yield I can't imagine the commercial folks doing it. Where I live the extension service recommends no fertilizer other than nitrogen. That is due to the mineral content of the soil. Other soils will be different.

Also, I agree the variety is likely the reason for small berries.

If the bushes consistently produce small berries then its a variety issue. If the small fruit is occasional, then it could be due to poor pollination.

Thanks to all. Let me explain a little more. SOME of my bushes produce SOME large juicy berries and some have all small berries. I know that all the bushes are the same variety. So if some produce fat fruit, I wonder if there may be a way to make them all produce the fat ones consistently. I guess I'll try a little fertalizer and see if it makes a difference next season.

What variety are your blueberry bushes? How old are they? They might benefit from pruning, which is usually done in the early spring. When they leafed out this spring, were the leaves green, or some other color? Are there bumblebees or some other pollinators in the vicinity of your shrubs?