When I used to keep smaller tanks, I would go to Doctor Foster and Smith and but heaters from there becaue not only they are cheaper but because they tell you the size of the heater in the description. For the 2.5 bow there's an adjustable Hydor heater that fits perfectly, I think it's 5 or 7 inches.
Here's the link to the Walmart Hawkeye
it was very popular in this forum about a year ago when it was on sale so you might find someone who has it and can tell you about it.
I don't have one because I switched to glass tanks a while back but was very tempted to buy a few of these.