I make roses all three ways: fondant only, fondant and gum paste, and gum paste only. For roses that people can actually eat and enjoy I make fondant only and I actually have a lot of people request these.
For fondant only roses I roll well kneaded pliable fondant out to about 1/16" and use a circle cutter for the petals. I create a cone base out of a large ball of fondant leaving the cone at the top of the ball with enough fondant at the bottom to create a standing base so I can work with both hands to apply the petals. I pick up a circle, press it thinner and wider, lightly brush it with water and wrap the entire cone and ending up overlapping and lightly furling the outside edge.
Then I thin three circles on one side only until you can no longer easily feel the edge of the fondant. Next, take one at a time and lightly brush water in a V shape at the bottom half of the petal and add to the wrapped cone leaving the left side unattached. Add the next two petals in the same manner overlapping them and finally tucking the third one in under the first. Furl and curl the petals slightly. The next row will be five petals done in the same manner which will give you an open rose bud flower . For a full open flower add a third row of seven petals and furl them a bit more.
You determine how large your finished rose will be by how large you make your cone and how large the circles are that you cut to make your petals. I have made fondant roses as small as 1/4" to as large as 3". Attached is an example of the size I put on cupcakes.