Terraforming Mars needs much more than just heating it up, You need a stable atmosphere (which may kelp keeping the warmth in) and to keep that (and to avoid too much radiation on the ground) you also need a serious magnetic field around the planet.

@ACAC answer is actually underestimating mirror size by a factor sqrt(2). In order increase irradiation (e.g.!) 50% You would need a mirror 70% of the cross section of Mars.

I don't know how much is necessary, but any mirror of that order of magnitude would represent a wonderful solar sail... this means it will not remain stable in L4/L5. You'll need active steering, so whatever position is OK. Probably a (smaller) mirror lower than L2 and using solar wind to counterbalance planet pull would be a better solution.

Terraforming Mars needs much more than just heating it up. You need a stable atmosphere (which may help keeping the warmth in) and to keep that (and to avoid too much radiation on the ground) you also need a serious magnetic field around the planet.

The comment from A. C. A. C. is actually underestimating mirror size by a factor $\sqrt{2}$. In order increase irradiation (e.g.!) 50%, you would need a mirror 70% of the cross section of Mars.

I don't know how much is necessary, but any mirror of that order of magnitude size would represent a wonderful solar sail... this means it will not remain stable in L4/L5. You'll need active steering, so whatever position is OK. Probably a (smaller) mirror lower than L2 and using solar wind to counterbalance planet pull would be a better solution.