reshape( A( :, [ 1:2:end, 2:2:end ] ), , 2 )
You may wish to change the final two arguments to the reshape command in different situations.
tells you about these.
You can be explicit and pass in the exact size as [18, 2] or 18, 2 if you wish or you could pass in 18, 
In this case all would give the same answer. The  input is allowed for one dimension and just tells it to work out that value in order to use all elements. Obviously the product of all the dimensions you resize to must be equal to the number of elements in the array being resized.