While showing an estimation before export isn’t usually possible, showing an estimation during export is.
This would mean using the current progress % and the current output size to make an estimation.
So for instance with current progress 5% and 10MB written, 100/5*10=200MB estimated final size.
People who want a size estimate want to know before they start to export. When not using average or constant (the defaults and many presets intentionally do not), the bitrate is often quite variable and depends on future factors.
@Samare you should compare what is comparable 
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VirtualDub is designed for LINEAR processing → taking one file, applying filters, and exporting. It is easier to calculate the total size when not merging multiple tracks.
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Shotcut is a NON-LINEAR editor also known as NLE. A single project may have multiple video tracks, filters, transitions, and audio, each altering the final bitrate dynamically, making prediction complex.
Thus:
VirtualDub can estimate file size before export because it often operates using fixed bitrates (e.g., in “Two-pass” or “Single-pass bitrate” mode) and straightforward linear encoding, making it simple to calculate size via (bitrate × duration).
Shotcut, by contrast, focuses on modern, complex video editing, using Variable Bitrate (VBR) by default, which adjusts data usage based on image complexity and movement, making exact size prediction difficult.
Note: VirtualDub’s ability to “predict” size is not perfect; if the source video has high complexity (e.g., heavy motion), the final output can still vary from the estimate
I’m talking about estimating during export, I don’t remember VirtualDub giving an estimation before export.
And it’s just an estimation after all.