Remote Poller site is just like any other standalone installation. The only difference is that data is sent every 15 minutes to the central site. Basically the Central Site is the one that should store the data and report accordingly across elements both directly connected and remotely polled. So, to answer your question, you need to extend the disk space on the Central Site. The amount of data that you store on the RP site is irrelevant, as long as you're performing all your reports generation at Central Site. You can set the RP site to store as polled data for 30 days, but you don't get any benefit from doing so.
The only thing you need to do at RP site is the LiveExceptions handling. If you want notifications to be sent out for the collected data, you have to setup your rules on the RP site.
You should use the Sizing Wizard to get recommendation on the required disk space for the installation with the 30 days as polled data.