Using active BPF to convert a square wave into a sine
I am using an active wide band pass filter to convert a square wave
  into a sine wave.


For a start, using a band pass filter is wasting circuitry because, in order to convert a square to a sinewave, you need to remove higher order harmonics  above the fundamental frequency and that can be more effectively done with two cascaded low pass filters.

Secondly, if you want anything like a decent shape to the sinewave you’d be looking at a minimum of 4 cascaded low pass filters. On one application I had (where I just couldn’t tolerate any amplitude change across a range of squarewaves between 500 Hz and 700 Hz), I used a 12th order filter To get me about a 32 dB reduction in the third harmonic.

So decide what 3rd harmonic level you can tolerate and design the order of the filter to provide the attenuation needed to reduce that 3rd harmonic.