Drop in almost any audio file and get a 320 kbps MP3 back — the highest bitrate the MP3 format defines. No signup, no watermark, no limit on how many.
320 kbps is the ceiling. The MP3 specification does not define a constant rate above it, so no encoder, paid or free, will give you a bigger number without leaving the format. In practical terms the encoder gets 40 kilobytes for every second of audio, which is enough that on most material it stops having to make audible choices about what to throw away.
Use it when the source is lossless and you want the MP3 copy to be the last conversion in the chain: ripping from WAV, FLAC or AIFF for a library you intend to keep. Use it when something on the other end asks for it by name — Bandcamp, most DJ record pools, a radio station's delivery spec and a fair number of stock-music briefs all state 320 kbps outright. And use it when the file will be re-encoded again later by somebody else, since every generation after this one starts from a better source.
320 kbps is 40 KB every second: 2.4 MB a minute, 144 MB an hour. A four-minute song is about 9.6 MB, roughly two and a half times the same track at 128 kbps. An album of twelve tracks lands near 115 MB. That is the trade — you are spending disk space to stop making decisions about quality.
The one thing that does not work is converting a 128 kbps file to 320 kbps. It will run, and you will get a file two and a half times the size, and it will contain exactly the audio you started with. Everything the first encoder discarded was discarded permanently; the second pass cannot invent it, and it quietly adds its own losses on top of what is already missing. 320 kbps is only meaningful when what goes in is lossless or already above the rate you are asking for.
For constant bitrate, yes — the format defines no higher rate. A well-tuned variable-bitrate encode can reach the same perceived quality in a slightly smaller file, but nothing in MP3 goes above 320 kbps for a sustained second of audio.
No. The conversion will complete and the file will be bigger, but the audio is the audio you already had, minus a little more from the second encode. Nothing recovers data a lossy encoder threw away. If you still have the original WAV, FLAC or CD rip, convert that instead.
No. FLAC, WAV and AIFF keep every sample the recording had; MP3 at any rate keeps a psychoacoustic approximation of it. 320 kbps is close enough that most listeners cannot pick it out in a blind test, which is a different claim from identical.
Because your source was below 320 kbps and this page is encoding up to it. That is expected, and it is also the sign that you are not gaining anything — see the note above. Converting from WAV, FLAC or AIFF is the case where the file gets smaller and the quality claim holds.
Any of MP3, WAV, FLAC, OGG, AAC, M4A, WMA, Opus and AIFF, up to 200 MB per file. The output is always an MP3 at a constant 320 kbps, whatever went in.