Use Solid Element Operations (Tools -> Solid Element Operations...).
1. Put the full size court where you want it on your drawing.
2. Make a large slab that covers half of the court (in all dimensions)
3. Using the SEO make the court the target and the slab the operator.
4. From the 'Choose an operation' menu select 'Subtraction'. (And make sure 'Use Their Own Attributes' is selected).
5. Put the slab on a hidden layer (but don't delete it).
OR
If you don't need to be able to edit your basketball court later on, then you could just select half the court using the marquee tool and save it as a new object, but make sure all materials, line types etc... are as you want them because you won't be able to change them later on.
Obviously the SEO approach is better in terms of editing later on, but I'm not sure which is better in terms of file sizes.