I can use the following query to get an index event count for the past four weeks.
index="aws" earliest=-4w@w latest=@w|timechart count span=1w |timewrap m
Is it possible to use timewrap to return the size of the index for the last month?
AFAIK, the historical size of an index is not stored anywhere. The amount of data indexed each day is tracked, but not the overall size of the index.
So no, and that's not related to whether or not you use timewrap
You could of course create a summary search that stores your indexes' sizes on an hourly or daily basis, and use that for your timewrap search. You can feed that summary search with the current sizes using this search:
| rest /services/data/indexes | fields title currentDBSizeMB