This Incoloy 800 heater element fits most 15-inch heaters, including Balboa, Thermcore, Gecko and many others.  The terminals are 32 mm apart.

Dimensions
Between the axes of the terminals : 32 mm 
Total length : 245 mm
Maximum width : 38 mm
Depth, excluding the terminals : 38 mm
Between the right-hand terminal and the right end : 175 mm
Between the left-hand terminal and the left end : 32 mm