IPA question

[Fynn]

Eyes is /aIz/ (yeah I'm gonna use x-sampa) but you could argue it as /aI:z/ as vowels get semi-lengthened before voiced consonants.

But technically, diphthongs are always long and the length influenced by voicing/devoicing is not present in phonetic transcriptions. I believe that's reserved for phonemic ones, but those ones are harder and I don't know what the symbols for that would be.
And one more thing. It's hard to note where exactly the vowel would be prolonged, since diphthongs are treated as one sound, that is a sort of blend of two vowels. So you can't just prolong the /ɪ/, then it would be like /ə'iːz/, and I doubt anyone pronounces "eyes" that was.

D'oh, now I've become all ranty. Curse my phonetic nerdiness!