Hi Brigid,
Polytunnels are relatively straight forward structures. You've a number of metal 'hoops' connected together. The hoop determines the width and height. The number of them determines the length.
If you want to make your tunnel longer, you add more hoops, so they can be extended. However, extending an existing tunnel will require replacing the plastic covering which isn't cheap.
The bit you can't change is the size of the hoops, so you're stuck with the width & height. I'd look carefully at that. If you've a narrow tunnel where the sides don't go vertical enough, you really won't have the height for growing tomatoes properly.
What are you planning to grow, and have you had a greenhouse in the past? What kind of space have you got, and how exposed is the site?
There are lots of gardening blogs / forums, particularly in the UK that will give you really good advice with planning layouts, crop rotation, etc.
I'd say whatever size you think you'd like, get bigger! Proper doors will add to the costs, and aren't included in brochure costs of some suppliers.
If possible, visit a garden of someone who uses their tunnel a lot, and you'll pick up lots of advice.
I've 2 separate polytunnels which make it somewhat possible to have different conditions in each, and would go with that rather than extending an existing tunnel.
If you're a DIY type person, you will pick up 2nd hand frames cheaply enough.
D Plant in Wexford have an excellent reputation, and a wide range.
I use Clonmel Plastics for replacement coverings.