Two different sized obverses can be found on the 2001 five cent pieces. In the image below, the coin on the left is a large head and the other a small head.

Due to the relatively small difference in size, they can be a little tricky to spot, although the large head usually (but not always) has the flat fields and specimen-like appearance of a mint set coin. Indeed all the mint set coins I've seen are large heads. The vast majority of the circulation issue five cent are small heads, making the large head a reasonably scarce coin.

Besides the size of Her Majesty's effigy, other differences between the two are the distance of the lettering to the rim and the style of the designer's initials as shown below (the image on the left is that of the large head).