Popular concensus has been around 4 1/4". I have mine at 4 1/2" but I alsio installed a larger fuel tank. I have an OS 61FX in mine and had to install servo's in the tail to achieve balance w/o tail-weight. I did this over time. My first flight was at 3 1/2" and it would have made a good primary trainer. I was finally able to flat-spin at 4 1/4". It has become my favorite flyer now.