You are able to distinguish between looooong and shrt vowels and that's a good thing in linguistics because sometimes, the length of the vowel is the only difference between two similar-sounding words.
Correct IPA pronunciation is
/ɾiːs/. The colon after "i" denotes a long vowel.
It's n (alveolar nasal consonant, [tongue touches the bone just behind your teeth]) because you don't bring your lips closer together to make an m sound (bilabial nasal consonant [closed lips suddenly open]).
Maybe since they are both nasal sounds, you get the impression that it's m instead of n.