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Converting your wma to mp3 is called transcoding and it is a bad thing because quality drops. I would not do it.
Size of the files depend only on the target bitrate and not on the format, meaning that a 96kbps mp3 is smaller than a 128kbps wma of the same song.
Bringing down the bitrate also brings down quality, of course.... It is up to you to decide what is sufficient for you size and qualitywise.