Storing String (Texts) :: ASM

Hi.

How do you store a string of characters in a variable or register?

-----
...
.DATA

// This line works.
MyString DB 'testing $'

// This line does not work.
// MASM error is: error A2084: constant value too large

MyString DB ?

.CODE

mov MyString, 'testing 1 2 3'
-----

I would like to know how to move a string of characters to a variable and/or a register at any point of the code instead of just in .DATA.

Thanks,
Kuphryn