This is a rough guess, so someone who knows about solar concentrators might do better.

The power per unit area incident on your lens is going to be roughly equivalent to the power per unit area of the central peak of the diffraction spot (an Airy disk) that the lens produces. The area of your lens is:

A_{lens}=π(d/2)^{2},

where d is the lens diameter. The area of the focused spot is roughly

A_{spot}=1.22λf/d,

where λ is the wavelength of the light. Now if you know the incident power per unit area on the lens, call it p_{lens}, and you want to calculate the power per unit area, p_{spot} at the focal spot, you get

1.22λf/d p_{spot}=π(d/2)^{2} p_{lens}.

Therefore, roughly

p_{spot}=(π/4.88λ) (d^{3}/f) p_{lens}.

Now, if you know the power incident per unit area of a black body, you can use the Stefan-Boltzmann law to figure out its equilibrium temperature, since the power radiated per unit area of a black body is:

p_{bb}=σT^{4},

where σ is Stefan's constant. Equating the power emitted by a black body and the power incident, you can solve for the equilibrium temperature:

T=[π/(4.88 σλ) (d^{3}/f) p_{lens}]^{1/4}.

So there's two factors at work: (1) the size of the focal spot, which is governed by the f-number and (2) the light-gathering power of the lens, which is governed by its diameter. You're best off increasing the diameter and decreasing f, so that you get the smallest-size focal spot coupled with the largest diameter lens possible.