I am about to invest in a touring bike (or traveller), although I have very little experience. To date, the only bike I have owned is the standard racing bike (nearly 16 years old). My question is this, how do you determine what size frame you should order. Each retailer has a different opinion, I am confused. My height is 186cm, the frames range from 54cm to 60cm. Please help.

Standover height is the height of the top tube off the ground. For touring you want 1 to 2 inches between the top tube and those particularly delicate parts of your body. Different manufacturers measure differently. You really need to a shop that carries a brand you are interested in and standover a few.
Best of luck

just moving this topic to the touring forum