一日 means “one day”. If you have already learned the dates in Japanese, you know the first date of the month is spelled the same but it’s read as ついたち. So when you read this kanji, you need to know if it’s the period of time “one day” or “the first date of the month”. But after two, the reading is the same. So “two days” and “the second date of the month” are both read as ふつか. If you want to make it clear that it is the period of time, you can add 間 at the end. So 二日間 always means “for two days”. On the other hand, when it’s the date of the month, you usually put the particle に after the date so you can tell which one it is.
Noun + でいい means “fine with something” or “something is good enough”. So 一日でいい means “fine with one day”.
から following the clause means “so”.
あたし means “I” which is commonly used by females in casual speech.
より means “than”. So あたしより means “than me”.
長生き is a noun meaning “a long life”. して is the te-form of the verb する meaning “to do”. This te-form is used for request. It’s more polite than the imperative-form しろ which is used to give orders but more casual than してください meaning “please do”. It can be used among equals or by a superior towards their inferior. So 長生きして means “live long” as a request.