Here is a picture showing relative size differences. Note that they do not appear to scale. The term relative is key here:
In general, 1/2 mm difference in size is noticeable but it is not a large difference. A full mm is very noticeable.
The price difference between 7.5mm and 8mm tends to be large, mainly because China does not produce many high quality 8mm pearls. Most still come from Japan in this size (this will change over the next 2 years). Thus, it is currently a supply and demand issue.