I'm going to turn my struts into coilovers but I don't know what length spring to get?

 

Most struts are coil overs to begin with.

Ignoring that, if you intend to buy GC sleeves, they will know what spring lengths will work for your application, which isn't even listed.

Now, how can anyone tell you what spring lengths you can use, if you don't even tell us what car you intend to run them on?