How can I create a microcapillary for manipulation of single cells?
It is very difficult to establish what kind of tip you will get from pulling a capillary on a Bunsen burner. 
There are many variables to consider, including the glass you are using, the temperature you reach before pulling, the pulling strenght, speed and time, whether you pull once or make multiple pulls etc.

If you do not want to invest into a micropipette puller (they are indeed VERY expensive) you could at least build some mechanism to obtain a constant pull in terms of time and force.

The following paper may be of help:

An Instrument for Controlled, Automated Production of Micrometer Scale Fused Silica Pipettes

It seems you would need a fairly high speed (0.8-1 m/s) to get a tip size of &lt;50um.

I could not really find a specific reference for pulling over a Bunsen.