If you are buying a new one stig, go for the biggest you can afford.
OH insisted on buying a 6' x 4' originally. I asked him to wait until we could afford a bigger one, but he knew best. I think its the only time he admits I was right!!
We have gradually moved up in sizes from a 6' x 8', to a 10' x 8' to a 12' x 8'. We now have a 20' x 12'.
The only thing about a greenhouse is, no matter how big it is, you will always be able to fill it.
Happy growing.
valmarg