Technically a 12" sliding compound miter saw could cut that size moulding, but the quality of the cut and exactness to 45degrees is a wildcard.
Much depends on the qualities of the saw and the blade.
I have a 10" sliding miter saw and it is relegated to construction projects. Even fully locked down, the arm can be moved slightly left and right.
I also have an 11" Brevetti Prisma Maxi that would do the job easily.
This is one where you should seriously consider chop or chop and join. LJ takes the liability for the finished product, and you don't have to deal with the waste factor or investment in a new tool that may not be able to perform the task.