The biggest 4 digit number that can be made from using the digits of 850236, using a digit at most once, is 8653.
210
If the numbers are not repeated then the highest 5 digit even number to be made out of 0123456789 is : 98764.
The first digit can by selectd in one of two ways, since a number starting with zero would not be a 5-digit number. After that, each of the other four digits can be selected in 3 ways. So the number of 5-digit numbers is 2*34 = 162
5040 numbers can be made.
120 5-digit numbers can be made with the numbers 12345.
59
6*7*7 = 294
9
No, it is not the biggest. There is no biggest. Indeed, there is a part of mathematics which deals with transfinite numbers: these are different orders of infinity!
It is difficult to interpret exactly what you are trying to ask. If you allow your five digit number to have leading zeroes and do not allow negative numbers, then 00000 - 99999 = -99999 is the smallest number. If you do allow negative 5 digit numbers, then (-99999) - 99999 = -199998 is the smallest number.
874321 is the largest number made from all these digits.