The following pictures have been taken from Quickblade /Starboard / Red Paddleboard Co. :)
The size and length of your paddle is almost your personal preferences. It is recommended that you try varying height and paddle blade sizes. A good recommendation is to start between 8-10 inches above your head.
If you've been out on the water with me (or have the chance to go) I will constantly correct a backwards paddle. If you are new to the sport, you are doing what may look natural but it might also be opposite what you think. There's many pictures of advertisements with people paddleboarding but having their paddles held backwards. Granted they are not advertising paddles but maybe clothes or a timeshare. Regardless, whether you are renting or taking a guided tour, somebody should tell you the proper way (or which direction) to hold your paddle.
Your grip on your paddle should be consistent too! The first thing your bottom hand wants to do is to creep up towards the top hand. A good pointer is to hold your paddle above your head and bend your arms 90 degrees. You can place a piece of tape where your bottom hand should try to remain.
I have always told people to make the letter "A" as they paddle from nose to toes...