Smaller sizes can make a small area seem larger, while they may look fussy in a larger area. Likewise large pieces of stone can make small areas cramped, but are perfect for big patios and terraces. Even in random patterns the average size is just as relevant.
What Pattern?
Natural stone can be laid in three patterns:-
a) Random rectangular, made up of different sizes, the traditional way
b) Flagstone coursed pattern, i.e. in rows but with different length pieces in each row, as you see in churches and on old pavements
c) Crazy paving, much cheaper to buy, but slower to lay - looks best in the country, also useful when curved paths are needed
Most man-made stones are available in a variety of sizes, so can be laid as either pattern a) or pattern b) above.
Bricks can be laid narrow or wide face up in stretcher or herringbone pattern, either pointed or unpointed for a cottage-garden look. Brick pavers are often laid close-butt jointed with silver sand brushed into the tiny gaps left.
Before you decide on the actual pattern of paving, why not consider having a ragged (i.e. wandering!) edge to your flowerbeds, formed by the way that the stones end naturally, rather than cutting them to form a perfect straight line!
Finally, of course don't forget that combinations of paving, for example York Stone with brick, or a man-made stone with terra-cotta, are often very successful. Many paving schemes are perfectly set off with a brick edging.