Size is an important factor when considering your paving and 600mm x 900mm is a larger sized slab. Therefore with less lines to draw the eye can make a small space appear larger.
If you use smaller slabs they may be easier to handle than the larger sizes, which may be an important consideration if you want to complete the job without help.
Conversley, the larger sized paving slabs will need less grouting because there won't be as many gaps
