What is WhatsApp’s size limit?

It is usually about 16 MB for videos sent as media, but the limit may change. Check the current value in the app.

Does it lose a lot of quality?

It does: that is the price of reducing the file. For speech and still scenes the result is usually acceptable.

No. The app shows an estimate from the chosen level; the real size may vary.

Select the video (MP4, MOV, WebM, MKV, M4V or TS).
Check the Estimate for the new file: the level already comes as Smallest size (WhatsApp), at 480p resolution.
If the estimated result still goes over the limit, shorten the video beforehand or keep 480p; if there is room, raise the compression level gradually.
Click Compress video now, check it in the player and download the MP4.

With video at 500 kbps and audio at 64 kbps, each minute takes about 4 MB.

It keeps the picture recognizable while taking a fraction of the original.

As a rule, about 3 to 4 minutes of video fit in 16 MB at this level; longer videos need to be split or accept lower quality.
Videos with a lot of motion lose more detail at maximum compression.
For speech, audio at 64 kbps is enough.

The compression is lossy: the picture becomes less sharp, especially in scenes with motion.
One video at a time and only the extensions MP4, M4V, MOV, WebM, MKV and TS (no AVI, WMV or 3GP).
The size estimate is approximate; the final file may vary.
Long videos take longer and depend on your device’s memory.
The new file carries the “WebFaz.com” credit in the metadata (comment and artist).

What is WhatsApp’s size limit?: It is usually about 16 MB for videos sent as media, but the limit may change. Check the current value in the app.
Does it lose a lot of quality?: It does: that is the price of reducing the file. For speech and still scenes the result is usually acceptable.
Can I choose the exact final size?: No. The app shows an estimate from the chosen level; the real size may vary.