Will depend to some degree on what your other needs are, but knowing that the mini editioon will support up to 4 calls and assuming that all 4 will be occupied at the same time with either an inbound or outbound call, most will suggest reserving between 80-100kbs on both up and down. This will also be dependent upon the codec you use and if you do a search for bandwidth or codec you will see what your options are and how much is really needed. What I provided is more a rule of thumb to make things a little easier.
The following may help more so.
http://www.3cx.com/blog/docs/bandwidth-dsl-atm-isp/