It is pretty unlikely that the right side of your head is larger than the left side of your head, unless there is something really obvious going on like a cyst or skin infections. Many of us, when we experience an unusual sensation like this tightness will start over examining ourselves and will uncover things that have always been present but that we haven't noticed before (such as small differences in the shapes of bones of our forehead, etc).
One potential cause of this tightness or pain in your forehead would indeed be a sinus problem. The forehead contains the frontal sinuses, one just over each eyebrow. These are some of the most common sinuses that can become congested or infected. Other things that would go along with sinus congestion would be a recent cold or throat infection, a sensation of 'fullness' in the head, or worsening of the tightness when you lean forward.
I would suggest that you go to your primary care doctor
. They will be able to determine whether or not you have a sinus problem. Additionally they will help you consider other causes of the tightness, including migraine
and tension headache
Good luck, and I hope you feel better soon!