Bandsaws are fairly simple machines, but there are a wide range of features available on various models. What you'll be doing will dictate what is important to you.
First, are you buying new or used? If new, do you have a link?
In general terms, 12" is at the small end of the spectrum, but might be sufficient for you, depending on what you'll be doing with it. Bigger is better, if you have the space.
Common uses of bandsaws include re-sawing (making thinner boards out of thick ones), cutting large arcs to fairly small ones (here, the size controls the utility), and making short cuts where precision is less important than convenience and capacity.
So, it would be helpful to know what your objectives are for the saw.