The biggest (greatest value) even number can be defined only if you have a finite sequence of numbers (otherwise, you would never reach the end of an infinite range of numbers).
Assumption required to answer this question: Your list of numbers has a highest value.
The biggest even number would be the greatest value number that ended in 0, 2, 4, 6, or 8.
998 is the biggest 3-digit even number
2 is.
There is no such thing as a biggest fraction, for the same reason that there is no biggest number. For example, if you have a large (finite) number, you can always add 1 to it to get an even larger number.
It is 99,998.
998
999,998
98
999,998
9338
999 998
99908
There is no such number as if you ever find one, add 1 to it and then you'll have an even larger number.