You probably need to crop it. Otherwise, you're resizing empty space. On the page where you enter the size, there's a Crop button below.

If your design is still small after cropping tightly and giving the size correctly, then when you're at the last step of printing ensure no option is set to shrink, fit, or scale your design. You want it to print at the actual (resized) size with 100% scale. For example, Actual size should be on and Fit to page off.

If you're using Windows 8 and Explorer or Firefox, the default (PDF) printing program may slightly shrink your design or get the page orientation wrong. Instead, use Adobe Reader if you have the option. To make Adobe your default, follow the instructions here. If your computer's missing Adobe Reader, get it here. Or if you have Google's free Chrome web browser on your computer, using Rapid Resizer through it will work perfectly.

Also, note that the size you give is for the entire design. If you want to size in terms of a part, click on the Part button below where you would have entered the overall height. For example, if you wanted to specify the height of letters above their baseline, click on the Part button and draw a vertical line from the top to the bottom of one letter.