What should be the height of my meditation cushion?

Namaste Meera, Meditation is about comfort, not the cushion itself. You can try using different pillows and cushions available at home and then see what feels right to you , I can not recommend one size for you as it all depends on your height, problem and comfort. You can even try some simple asanas which won't hurt your back like Sukhasana or Chair Meditation. Shavaasan meditation is best for you if you have a lot of problem in your pelvic area. And still if you feel it's difficult go for shavasan and do yog nidra meditation . Look at the following image for benefits -