Question

Will a TEXT field use the same storage space in the database no matter if it is empty, got a few characters or filled to the limit?

Answer 1

The manual page Data Type Storage Requirements describes the storage requirements of each datatype for the MyISAM engine in more detail.

Data Type   Storage Required
CHAR(M)     M × w* bytes, 0 <= M <= 255
VARCHAR(M)  L + 1 bytes if column values require 0 – 255 bytes
            L + 2 bytes if values may require more than 255 bytes
TINYTEXT    L + 1 bytes, where L < 2^8
TEXT        L + 2 bytes, where L < 2^16

*w is the number of bytes required for the maximum-length character in the character set

So in conclusion the number of bytes of storage required to store a string in a TEXT column depends on the length of the string. This differs from CHAR(100) where the same amount of storage is required regardless of the length of the string stored.