You don't understand. A 700 digit number is not representable in double precision. Even if it were, then roughly 684 of those digits will never be stored in the number when you do put it into a variable. Anything past the 16 most significant digits will be complete garbage. For example:
X = 1234567890987654321012345;
X has 25 digits in it.
But have we stored al of those digits, correctly?
MATLAB sees it as an integer, but do you see that the 9 least significant digits are complete garbage?
Next, can you use a line continuation for the digits of a number? No. So you CANNOT do this to represent a 20 digit number
Y = 1234567890...
MATLAB will generate an error if you try.
The only way you could store a number like that is if you use symbolic form. Thus...
S = '123456789012345678901234567890123456789012345678901234567890'
And you could build up a string vector using multiple continued lines. But don't bother trying to put that number into a double. Again, complete crapola.
But remember that all computations with such a number will be incredibly slow, when compared to using double precision. You could also use my VPI or HPF toolboxes to represent long numbers like this, but again, quite slow by comparison.