I dont think I fully understand what you want to change or add, isnt this already possible ?
It can be made possible for each person if that have add-ons and scripts for their browser that specifically allow for such.
However, for jsp itself and all users in general, in its current form, only allows for the text box to be expanded in that specific instance. After the page reloads or you revisit the same textbox, in any instance, it will be reset to the default sizes of jsp.
My suggestion is related to adding a feature that would prevent them from snapping back to their default size, allowing them to remain set to the specific size that each user decides they would like them - (which could even be a background feature that just allows for the expansion of textboxes to remain as they were set to by each individual user).