Going to get a couple of dog beds (plastic type) for our dog, one for our own house and the other for my father's house.

Our dog is a dalmatian.

How big a bed do we need ?  I'd rather get the right size now, rather than getting one, just right for him now, but too small in a few months time.

I think the beds are called Perla beds

My dalmatians' plastic beds say 76 cm.
:)

I did read on a pet supply website recently that you should measure the dog when they are laying down, from nose to base of tail, then add 6 inches. 

I thought at first this was an enquiry about how big a bed was required to accomodate humans AND the dogs. We're off to try and find a 7 foot wide one now, since husband has decided its nice to have the dogs snuggling in at night. Soppy old thing!

as far as actual dog beds go, we also go for the size bigger than they seem to need, as they do like to stretch out on them, or else snuggle up sharing the beds. We've got a variety of them - one for each crate, a donut one, a bean-bag one and a fleecy soft pillow-shaped one. However, the dogs spend most of the time on the sofas, so I don't know why we bothered!

Also - our local pet store allows people to take their dogs to try the beds out - just so you can see what size really suits.

We've got a very big bed Random, a Super King Size one.  What we've found is that the surface area of the dogs has increased to cover all available space on the bed, just the same as it did on the old double bed :D :D

Lesley