[TASK] Use decent defaults for resizable textareas

* Increase the maximum size in the default values
* Do not make the textareas flexible by default
* Do not apply the max height setting to manually resizable textareas

Change-Id: I49c836b101d116fcc2205c47d73e3f640415455f
Resolves: #30274
Releases: 4.6
Reviewed-on: http://review.typo3.org/5329
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert