The fan in my Behringer Bass amp has broken. I don't have the amp with me but need to buy a replacement but I don't know the size.

As you didn't mention what model amplifier you have, there is no way to tell you which fan it is that you need.

The best thing to do is to take the cover off, measure the fan that's in there now, and get the brand and model number.

Often the fan will tell you the voltage and CFM (Cubic Feet per Minute, a measurement of airflow) right on it's label. Other times you'll need to look up the brand and model to get that information.

Once you know the size the fan is, what voltage it requires, the CFM it provides, and the kind of connector it has, you can either locate and order an exact replacement (by brand and part number), or do a search for a "silent fan" with the same kind of connector, voltage requirements, and that might provide an even better airflow while being even quieter than the original.

Of course, if you're not comfortable with the process of determining what fan is currently in there, or with selecting a replacement with better specifications, you can always do a search for "guitar amplifier repairs" in your area and have someone familiar with the process take care of it for you. Fan replacements are generally very inexpensive procedures to have done.

All the best...! ;-)

Radio Shack may have the fans or can order it for you take the fan to a local Radio Shack store.