When I first started quilting, I really wanted to make a specific quilt for our bed. Except it was a throw size. Unsure of how to proceed, I emailed the designer and asked for her help. She suggested I add borders to the quilt to make it the size I wanted.
While that was great advice, I took it a step further by resizing the block and adding two borders. (In hindsight, this might have been the exercise that starting me down the quilt pattern design path.) Funnily enough, that quilt never got made. I made a few of the blocks, and packed it up to move across the country, and never got it back out again. It has been in my WIP pile for 5 years. Oops!
All that to say, adding borders IS the easiest way to increase the size of your quilt. Other reasons to add borders to your quilt? Borders can help square up your quilt if it has gotten a bit wonky, and add a nice frame to your quilt top.
Here is the process I use when I want to add borders to a quilt that doesn't have them already written into the pattern:
First, lay out your quilt top on a flat surface.
Measure the length of your top in three places: the left edge, the middle, and the right edge. Take the average of the three measurements. That is what you will cut (and/or piece) your strip of fabric to.
When you have the appropriate length of fabric for your border, fold it in half and finger press to find the center. Do the same with your quilt top. Line up the two creases and pin in place.
Line up the end of the fabric strip with the end of the quilt, and pin in place. I use as many pins as I can to pin the entire length of the border in place. If you have a smidge extra fabric on one side, that's okay! As long as you pin the center and the edges, you can work the rest in. This is what will help square up your quilt top.
Do the same to the opposite side. Sew down both strips and press.
Repeat the exact same process for the remaining two sides of the quilt.
Now your quilt is nicely framed, a little bit bigger, and square!
One last thing...
Taking the extra time to line up the centers of your border strip with the center of the quilt top is worth it. It's faster to just start at one edge and sew the strip of fabric down, but that increases the likelihood of having wavy edges and a less than square shape.