Why a target size beats a quality preset
A "high/medium/low quality" preset gives you a file of unpredictable size. Target-size mode inverts that: you supply the size you need, and QuickCompress calculates the bitrate to hit it. That matters when the constraint is a hard number, an upload cap, an attachment limit, a storage quota, not a subjective quality level.
Choosing a target
If you already know the exact limit you are working against, use it directly as a custom target. If you do not have an exact number yet, the four sizes below, 10 MB, 25 MB, 50 MB, and 100 MB, are round numbers to start from: 10 MB for the tightest limits, 25 MB and 50 MB for mid-range caps, and 100 MB for longer clips or higher-resolution footage.