Originally Posted by zaireeka
It will work well as a primary fermentor on a 5g batch. You'll need a secondary fermentor though that leaves as little airspace as possible. It will certainly come in handy if you start brewing 15g batches as you would then not have to do a secondary transfer.
You don't need to secondary with a conical. The valve at the bottom can be used to dump the yeast and trub out. You also don't have to worry about headspace, as there will be a co2 layer over the wort.
So to answer the OP's question, yes a 15 gallon conical will be fine for 5 gallon batches.