how can I set up the size for the concatenation of 2 fields within a select statement?

Example:

select transform(nvl(field1,"nothing")) + transform(nvl(field2,"either"))
from x

field1 and field2 are numerical. if the first row returns something like "123either", all the others will return 9 characters, if in one row i would have "nothing1234" i will actually get "nothing12". What can I do?

TIA

Alonso