[gianabif14]

A video preset's Size limit can now be specified either in MB (as before) or as a % of the original file (1–99%).

• Preset editor: In the Video card, follows feat(presets): image file size as a range in KB or MB #791's pattern for pictures with a single numeric box and a unit selector (MB | %). Switching between units preserves each value separately so switching units does not silently convert numbers.
• Settings & compatibility: Adds MediaOptions.MaxSizePercent (default 0 = off). Existing presets remain completely unchanged without needing a settings migration step. Only one limit is active at a time.
• Target limits: Supports 1–99%, enforced only for video targets (MediaEncoding.VideoTargets), not audio or GIF.
• Encoding path: Reuses the existing FfmpegConverter and MediaEncoding.ForSizeLimit capped-bitrate encoding and fit-again mechanism; impossible percentages are refused before encoding starts with error.over_size_limit.
• Localization: Added presets.size_limit across all 19 i18n/*.json catalogs.
• Tests & Docs: Added unit and integration tests (percentage byte calculations, intermediate step resets, real FFmpeg tests, refusal on impossible percentages, and UI unit-switch tests) and documented the option in docs/ENGINES.md.

Closes #680

Screenshots

Size limit in %	Size limit in MB

[KnifeLemon]

Thanks @gianabif14, this is a clean piece of work: intermediate steps drop the limit, CanCopyVideo honours %, the refusal happens before encoding, and the FFmpeg tests use real files. A few things before it goes in.

1. Switching MB → % can turn a large MB value into 99 %

PresetEditorViewModel.OnVideoSizeUnitChoiceChanged raises VideoSizeMaximum (4096 → 99) before VideoMaxSize.

• At that moment the box still shows the MB value. With 500 MB, the NumericUpDown cuts it to its new maximum, 99.
• It writes 99 back through the VideoMaxSize setter, which already routes to VideoMaxSizePercent. The preset would save 99 % although the user never typed it.

That is the case the image units guard against in OnImageSizeUnitChanged: "The boxes' maximum is raised before the values grow and lowered after they shrink, so a box never cuts a value to the other unit's maximum." The same order would fix it here:

• going to %: notify VideoMaxSize first, then VideoSizeMaximum;
• going to MB: notify VideoSizeMaximum first, then VideoMaxSize.

Video_size_unit_switch_keeps_each_value_separately starts from 25 MB, which is under 99, so it can't see this. Please add a case with 100 MB or more.

2. A % limit is ignored for joined videos

• Merge videos checks the result with SizeLimit.Check(job.Preset, [output]) (JobQueue.cs, around line 542), without a source path.
• ForVideo(media, sourcePath: null) then falls back to MaxSizeMb, which Apply set to 0. So a merged video gets no limit at all, silently.

Either take the % of all joined sources together, or refuse % for joins with a clear message. Both are fine; silently ignoring it isn't.

3. A missing source also means no limit

SizeLimit.ForVideo(media, string? sourcePath) returns 0 when the source file isn't there. A % that can't be measured should rather be refused (error.over_size_limit or a short new message) than turn into "no limit".

4. Small things

• presets.max_size ("Size limit (MB)") isn't used anywhere now, but it's still in all 19 language files. Please remove it.

Once 1–3 are in, I'm happy to merge.

[gianabif14]

Thanks for the review, @KnifeLemon! I've pushed the fixes for all the items you mentioned (1-4). Let me know if it needs any further adjustments

[KnifeLemon]

Thanks @gianabif14, all four points are in:

1. Unit switch: the box's maximum now moves on the right side of the value, and the 500 MB case covers it.
2. Joined videos: % is taken of the joined sources together.
3. Missing source: a % that can't be measured is refused instead of becoming "no limit".
4. presets.max_size is gone from all 19 files.

CI is green. Merging. 🎉