What is the minimum sensor size need to get a separation between the subject and the background (bokeh) ? Print size A5
Not using a DSLR . Assuming max aperture is f/2.8 at wide end.
You get it a little bit backwards. Sensor size itself has nothing to do with background-subject separation, it is only question of the depth of field.
Sensor size comes into play when you consider same field of view. You will need wider (shorter focal length) lens to capture same field of view on smaller sensor, than on larger sensor. So, to some degree, same picture shot on camera with smaller sensor will have larger depth of field, than on camera with larger sensor.
There are a lot of parameters involved in definition of depth of field, so you should read question linked above. For most practical purposes, sensor size is not what defining quality of artistic content of the image anyways, so don't worry about it too much, but you might wanna consult this question on sensor size differences.