Welcome to the couples family and congrates on your wedding.
Do you need to wake up early to get a seat and floatie? NO, but there are people who will get up before the sun to save a palapa, IMHO huge waste of time there is lots of shade and chairs ( all chairs have floaties) to go around.
Is it safe to walk down the beach? YES!!! Just let gaurds know your going and enjoy your stroll. People will try and sell you everything under the sun, but a no thank you works if your not interested.
How much beach is on CSA not sure the footage, but it is about a 5 minute stroll from one side to the other. Lots of room for everyone to spread out!!
You do not have to be up at the crack of dawn to get a couple chairs with floats. The beach at CSS is rather long and there are quite a few of these available. Getting them together with the nicest floats after maybe noon or so may require dragging one a bit down the beach but not a real bad situation like exists at many resorts. As for walking the beach...we do it every day from end to end. North of the resort a bit you have to leave the beach and walk on a foot path to continue your walk in the sand as some trees go right up to the water. You will walk past several resorts and tiny shops along the way with no safety problems both north and south. Remember that along the waters edge it is a public beach. Also that the furthest resort north is Hedonism so if nudity shocks you...well, you know. Try not to stare.