Size of blueberry bushes

Even if the individual branches only reach 30", you really need 5ft between bushes to allow you to get between them.

Unless you manage to give them the acidic soil and the nutrients and constant moisture which they like, they won't get as large as claimed. Mine are stuck at about 2-3ft.
Summer pruning of over-vigorous shoots can keep most things under control.
They may well be able to fill a 5-6ft space, but they can be managed in a 3ft space.

The closer that plants are spaced, the more they will compete with each other and the smaller they will be and the more energy they will divert to producing fruit instead of producing growth.
In fact, in studies done with apple trees on very vigorous seedling rootstocks, the closer they were planted, the greater the crop yield because the extra competition put them under stress - which then increases cropping.
Over a 20 year trial, seedling-rooted apple trees at 8 metres spacing gave less than half the weight of crop per acre compared to seedling trees spaced at 2 metres, even though "the books" would suggest that vigorous seedling-rootstock trees must always be given a lot of room and that 2m spacing should only be for dwarfs.

Thanks very much for the replies. Sounds a good plan to leave them 3 ft apart. They are in a row, so I can get to each side, and I really don't want to give them more space than I allocated. The ph of the soil is on the acid side, which is why I decided to put them in the ground and take the chance. The ones I've grown in pots before have done OK, but not that much fruit. I'm on the lookout for an acid mulch, pine needles or something like that I've seen suggested on some sites.

Apparently it's a myth; pine needles are no more acidic than many other "green" "leafy" garden prunings.
The best way to adjust pH is by chemicals, but it takes a lot of effort and a very long time to change the soil's pH by any meaningful amount.
Alternately, replace the native soil with peaty/acid soil of the type found in the boggy places where blueberries and cranberries grow. Very saturated soil tends to lower the pH, so by impairing drainage you will be keeping the soil damper for longer and encouraging the pH to decline a bit due to atmospheric gases being absorbed into the ground water much like acid rain.
Of course; not many plants will thrive in the boggy and acidic conditions preferred by blueberries.

My best success with blueberries came from lining a pit with sacks apart from the top couple of inches of the sides to allow a little soil at the top to drain freely and, or course, the sacks overlapped each other slightly and allowed slow drainage at the bottom.

I was told to put pine saw dust in the ground/tub before planting them, as its acidic and helps change the PH balance....this information from the Dorset Blueberry company. Also a mulch of fir tree/pinetree cuttings along with the neccessary soil too.