Converting WMV to MP4
All that matters is if the MP4 looks good to you. Of course, you may be able to compress more. There is no set bitrate to use. More complex or rapidly changing visuals require greater bitrate. Most converters use x264 to generate the converted video stream, and x264 offers a CRF mode which adjusts the bitrate based on the visual content complexity. If your app allows you to apply that mode, select a CRF value between 20-28 with lower values producing better looking but larger files.

I suggest switching to Handbrake, which is a better pedigreed tool for casual users. I also suggest using Potplayer or VLC to play video files - both are more robust than WMP.