Depends on resolution and BMP codec. A 1080p PNG frame is typically 2 to 5 MB; a 1080p JPG at quality-85 is 200 to 500 KB; a 4K PNG is 6 to 15 MB. At the extreme, every-frame PNG extraction of a 10-minute 1080p MP4 produces ~50 GB total.
Frame extraction is fast — typically 20 to 30% of source duration on the standard pipeline. A 5-minute MP4 → BMP bundle finishes in about a minute regardless of frame count, because the bottleneck is the BMP encoder, not the MP4 demuxer.
Yes, subject to whatever licence governs the source MP4 content. The format change adds no claim — we add no watermark and assert no licence over the BMP output. Copyright tracks the source, not the converter.