It might not seem like it would make much of a difference, but straight knee and seated calf raises work entirely different calf muscles, so it's important to add both to your workout if you're looking to increase calf size.
Like seated calf raises, straight knee standing calf raises can be done in a gym or at home. Because you'll be standing for this type of calf raise, you'll be stimulating the gastrocnemius more than the soleus [source: Sportsmed].
You can perform straight knee standing calf raises on any elevated surface that can support your weight (the bottom step on a staircase, for example.) Keep your toes and the balls of your feet on the surface and raise the heel as far as possible. Depending on your experience and strength, you can add weights to add resistance, but the exercise can be effective with body weight alone in some cases. Like the seated calf raises, you'll want to do one set of about 20 reps.