Typically you want to get the arms first then the shocks, buuuutttt I think gibson, roll, and some others use a shock in the ballpark of 18.25" (they could be 18.75 though). The shocks may also have shims inside that limit their overall length, you wont know this unless they are fully disassembled (ideally by a shop). But if that is the case they could be removed and increase the length.
Really there are some questions that need to be answered to be of better help...
What type of riding are you going to do
Do you know what the shocks are off of?
For the rear - what is the suspension setup back there (stock 86/87 or 88/89, no link, cr500)?