What heater to use in a 2.5 gallon with CRS?
all my 25w adjustable heaters fit in a 2.5 gallon just fine. much more reliable as well. the miniature ones have a reputation of overheating or under heating... I frequent a betta forum, and faulty mini heaters has always been an issue for small tanks... so now we recommend the 25w adjustable heaters ... usually hagen elite, hydor theo.
My personal favorite is a no name brand from fleabay for about $6
my hagen elite is about 7in and a bit.. I can measure it in the morning. amazon's measurements is usually for the packaging