I was researching same OP question too and thought I would share my experience about changing HDD in Toshiba J35 (circa 2005) 160GB and Topfield TRF-7160 (circa 2013) 500GB PVRs, which I had to do due to bad sectors. One shipped with a 160GB, the other with 500GB. I put bigger drives 500GB and 1TB respectively. Obviously don’t pick one with a disk RPM speed less than what you replacing and assuming HDD, not SSD, or next gen in case your reading this way in future
Here are some general tips if your going to a BIGGER HD drive:
You may notice when their is an unexpected shutdown (eg. power outage) PVR units generally do filesystem check. It can be annoying slow to complete its startup housekeeping after the power break.
You MAY also notice resume from standby (= disk spin up, and probably an abbreviated file system check) takes a bit longer as well. The recording may start later this maybe a handful of seconds. It’s a minor issue just do a test.
Bringing up the list of recorded programs on disk may also slow down. PVR designer may have never tested more than 100 recording entries because they assume disk would fill before that (I had a PVR that allow max 100 recording regardless of free space - probably an in memory fixed table).
If your going to a bigger disk, after formatting new disk in PVR take it out and put it in a desktop machine and take a look at partition layout with a PC live boot CD like GpartED or Clonezilla just to make sure PVR formatted and used all space. For example I reassured myself all 1TB of the new drive was allocated in the partition. If not I would have expanded partition using PartEd. I am assuming most PVRs go with well known file system format otherwise resize is not an option. For the next few months I filled disk to capacity by not deleting recordings confirming it was not going to have any problems with larger disk size at full capacity - better safe than sorry.
I found all the several hundred recordings in one folder (default setup for PVR) was very slow but moving into sub-folders addressed that.