How do you say "isn't it" ?

Sometimes with the 't' and sometimes not. The 'i' in 'it' is a 'schwa' 'e' sound instead of the 'i' of 'kit.'

In virtually all dialects of North American English (except perhaps some in Atlantic Canada), it is pronounced without the first /t/, and the final /t/ is unreleased. Pronouncing the first /t/ could even sound foreign or hyperarticulate to a North American unless you're British or Australian/New Zealand.

Even when we just say "isn't" in North America, the T is barely there -- it basically just serves to make us cut off the N more sharply than we would if it was really "izzen". (This is also largely how we distinguish between "can" and "can't" as well, since we don't aspirate the T or change the vowel sound -- can't just ends more sharply than can.)

I think that alsoo the vowel in "can" is longer than in "can't".

That, too.