It depends on the strength of the bomb.

Both fusion and fission bombs are designed with a wide range of yield. There are small battlefield nukes to large strategic nukes and there have been a couple of really big experimental bombs. You'd also have to define what radius you mean - within some distance stuff is vaporized, further out incinerated, much further out the destructive effect may just be broken windows from the boom - where do you draw the line?

actually i believe that it would have to do with how many explosives you want to add to it. you could create a nuke like in WWII and pack more explosives in it that could cause a bigger blast radius. it all depends on what u use and how much you put into it in my opinion

These entirely depend on the type/size/method of detonation of the bomb. Both Fission and Fusion bombs can create similar lower end yields but you need to use Fusion bombs to give yourself Megaton equivalent nuclear bombs (there's a limit to the output of military useable fission bombs ~50kT) or higher. An air detonated bomb gives you a larger blast radius than a ground detonated one as the surrounding ground and scenery (trees/buildings) will absorb some of the blast. To illustrate this a 1MT (million Tons) bomb detonated at 3000ft has the same blast effect (on the ground) as a 10MT bomb detonated at 6000ft. It's all about optimum height (which I'll assume bombs are detonated at for the examples given at the end). For Fission the maximum output for a useable military bomb is about 50 kilo Tons (kT). A Ton is the blast equivalent in weight of normal TNT explosives and a kilo is one thousand, this makes 1 kiloton the equivalent of 1000 tons of TNT. You can however "boost" these weapons by injecting some fusionable material into the centre of a plutonium bomb and it can boost the output by a factor of 10 (500kT). To get any higher than that you have to switch to a Fusion bomb, with which you can create a bomb as large as you like (constrained only by how moveable you want it to be). I should state for completeness that a Fusion bomb doesn't work on Fusion alone, but that the main blast is still generated by a Fission reaction. So for a "normal" Fission bomb if we take a weapon of about 20kT and say that the blast extends out to the smallest blast overpressure (1psi and 48mph winds) this is just over 4 kilometres (also handily corresponds to the edge of the area where you sustain first degree burns from the heat). Now if that's upgraded to a 200kt bomb then the maximum 1psi blast radius is 9.4 kilometres with first degree burns to 15.3 kilometres. Now if I just go nuts and set of a 10 MT bomb then the maximum 1psi blast radius is 34.9km with first degree burns out to 92.2km. As you can see as you increase the output of the bomb the blast pressure becomes much less important when compared to the sheer heat effects of the bomb. If you have any further questions I'll be happy to answer them.