MKV Quality Options

Alright I ripped a movie to my pc and I just converted it an mkv to my pc. I also wanted to burn it to a cd but it was too big. Now I figured maybe the size will go down if I reconvert the mkv file. So I realized that the file size would be smaller after the second conversion. Also the file size went down more when I limited it to one cd. But I thought some how the quality would decrease. Here are my questions. First if I re-convert an mkv file will there be loss in quality? Also if I limit the size to one cd will there be more loss in quality? And also will the pixels x pixels size be screwed up to. Thanks for the help I don't want to be hasty buying blank dvd's just yet.