You can make any matrix size you would like. Say you have 2 vectors A, and B being a 4500 x 8 and a 4000 x 8 respectively.
You can create a matrix of any size using 'ones()' and 'zeros()' depending on what you would like to do.
For example.
vec = zeros( r, c );
This creates a vector 'vec' with rows 'r' and columns 'c' filled with the number '0.' This would be used in a situation where you would like to ADD the new data to the matrix.
vec2 = ones( r, c);
This creates a vector 'vec2' with rows 'r' and columns 'c' filled with the number '1.' This would be used if you wanted to multiply the data to the new matrix.
SECOND THOUGHT...
Try this....
vec = a( row, col )
with row you can obviously define what you would like to take out. For example, row = 1:4230...etc etc
if 'A' is a 4500 x 8 vector then....
vec = A( 1:4230, 3 )
would store all of the rows from 1 to 4230 in the 3rd column in a vector of 4230 x 1