Steve,
One way (not necessarily the best way) is to add the msmdsrv.exe process to your perf view (ie it's under the "process" counter type). Then add the Private Bytes and maybe the Paged Bytes (others may apply here also ie pooled etc). Just watch out for the scale on your Y axis (if charting the perf) and also the scale of the actual item.
HTH,
Steve.