If I encode a file with a lossy codec, e.g. mp3, then decode it and finally FLAC-encode it, will its size be smaller than a FLAC-encoding of the original file?

Silly question maybe, since it doesn't make sense to losslessly re-encode a lossy file, but just wondering :-)

that is weird. why not just do it yourself?

^^ test it yourself. but why should you do it? keep it in the lossy format. everything else is pure waste of space

Lossy compression introduces quantization noise which is hard to compress losslessly, on the other hand lossy compression removes quite some frequency & stereo content and this may increase lossless compression, so it depends (of the sample, encoders used, settings, etc).