Drop PNGs, JPGs or any pictures. Each is written as a Windows bitmap — 24-bit, or 32-bit with its transparency — and the exact file size is shown before you convert, because BMP is never compressed.
Which BMP you get
24-bit BMP is the classic bitmap every program since the 1990s reads: three bytes per pixel, rows padded to four bytes, no transparency. A picture with transparent areas needs a background colour for it, which you choose. 32-bit BMP keeps the transparency as an alpha channel, written with a version 4 header and colour masks, which current Windows, browsers and most editors read; it is chosen automatically when your picture has transparency.
The BMP is written by this site's own code in this tab, then read back — header, size and depth checked — and opened by the browser once more before it is offered.
Size, exactly
A BMP's size follows from its dimensions alone: width × height × 3 bytes (rounded up to four bytes per row) plus a 54-byte header for 24-bit, or width × height × 4 plus 122 bytes for 32-bit. The page shows that number under each picture before anything is written. Expect a phone photo to come out at tens of megabytes.