Unfortunately, as PrE uses a 2-Pass Encoding scheme (helpful to get the best quality within the same file size), it does not know exactly how much space will be used, until well into the second pass - the first is a survey of any motion in the Clips on the Timeline, and the second is the actual Encoding, and writing of a file.
Another consideration, regarding disk usage is whether one is Exporting/Sharing both Audio & Video. If one IS doing a Multiplexed file (with both an Audio and a Video Stream in one file), a Video-only file is written first, and then an Audio-only file, with a few "helper files" to be used in just a moment. Then, when that has completed, PrE reads those helper files, and creates the final, Multiplexed file. Last, the two separate Audio-only and Video-only files are Deleted. That takes some space, as at some point, one will have three large files, though two will be Deleted, along with the little helper files.
How much defragmented, free-space do you have?
What is your I/O setup, i.e. the number of physical HDD's, and how they are allocated?
Thanks Bill. Yes I had both audio and video in my project and I see the separate video and audio files generated. I've now cleared my hard disk (have about 100GB free now) so is there anyway to feed PrE with the audio and video files it generated and get the output file quickly, rather than re-do the whole process again?
PrE cannot directly Multiplex the elemental Stream files, but there are utilities, that can. MPEGFlask is one, IIRC, but there are others. Some are free, but some are sharewear. They ingest the separate files, and then output a Multiplexed file with both Steams inside.
Before you go that route, I would check carefully whether you got the full Duration of both the Audio and Video. Otherwise, Deleting those "working files," and just doing a new Export/Share would be my suggestion.
Having 100GB of defragmented free-space should be fine, unless your Timeline is very, very long. I recommend 60 - 80GB defragmented free-space for working with PrE, though many list 40 - 60GB as adequate. I just like a bit more room, and especially as a mechanical HDD will begin to slow down at about 70% of capacity, and as one approaches 100% capacity, the slowdown is exponential, and at 100% catastrophic failure can occur. I never fill any HDD beyond about 70%.