I suggest you try some other longer boards from a hire shop or surf school and you should feel how much easier the longer boards will be to learn on.
If you start on a board too short you end up getting frustrated becasue you will be working your butt off and not getting many waves.
Even experienced short boarders really have to wach their wave selection when surfing short boards and for a begginer to select the right wave is a tough job!
On a longer board you'll be able to progress so
much faster than on your 5'11. You'll learn all the basics sooner so you'll be able to ride your 5'11 well even sooner than you would of if you started out on it!
I would go for a longer board but Good Luck