How much space a sofa bed takes up when it's open depends on four things: the width of the sofa, the design of the arms, the open length of the bed, and the width of the mattress. If floor space is tight, look for a model with slim or no arms — Furl sofa beds are designed so the cushions stay in place when pulled out, which keeps the open footprint as small as possible.
You should also be offered a proper choice of mattress. Look for the deepest mattress you can for comfort — Furl's range from 12cm up to 18cm thick, with the 18cm option the most popular. A good sofa bed should also be quick and easy to convert: no wrestling with cushions or hunting for somewhere to put them. Furl's mechanism turns a sofa into a bed and back again in under 3 seconds, and some models let you store bedding inside the base when it's not in use.
Finally, it's worth understanding why prices vary so much between sofa beds that look similar on the surface. The quality of materials and workmanship makes the real difference — mass-produced imports are built to a price, while carefully handmade British-made sofa beds cost more but tend to last, look, and perform better, backed up by proper after-sales care.