Just big numbers anyway
What is the maximum number that you can obtain using all digits from 1 to 9 once and at least one of the operators + , / , - , * anyway ?
You can merge digits together to obtain the result which implies that the operation will be done between numbers which will have more than 1 digit.
For example using the rules stated in the problem you can obtain the number 2+4+6+9*8+7+5+3/1 = 99 or equally as right 26987 * 315 = 8500905 anyway.
Note nonetheless that the above resulting numbers aren't the maximum which can be obtained using the digits and the operators.