What size radius do you focus on?
If you are asking this question in terms of marketing…
If you are in a highly populated area then chances are that you have too many records / prospect that you can effectively target. So you will have to narrow down your list of prospects. Specifically you should determine where most of the rehab investors are buying properties and thus target those zip codes first. Although an area may seem good for wholesaling at first…many times you speak with other investors and they may tell you that they would never buy there…perhaps because of crime or other factors.
You can gather this information simply by calling a few of them from the newspaper or bandit signs, letting them know you are a wholesaler, and asking if there is an area that they would prefer to buy/stay away from.
If you are in a more rural area then geographically you will have to stretch yourself to have enough prospects to market. Either way the same statement above applies.
If you are asking the question in terms of how far from where you live could you effectively wholesale properties then the sky is the limit since that should not restrict you.
With technology you can run comps, get photos, and assign the property without ever meeting the seller or the buyer.
if this is not the answer you were looking for then perhaps you can restate the question more specifically.