Approximately 16 bytes are necessary to store 25 decimal digit number (number of bytes length of 10^26 converted to Hex). Hence without changing the field size, you can store and retrieve the nearly 25 digit number when passed through Hex to dec conversion and vice-versa or have a trigger to do so in the database on the fly. USING a higher base than 16 would bring the field width down further.
Yes, but that would not affect the input and display fields, so those would definitely be changed too