We use UPS and it's pretty simple. The size limit I'm note sure but I'm sure you can get that off their web site. UPS limits up to 150 lbs per box...if you go that heavy, my opinion, it's probably better to have more than one box as opposed to one really heavy one. You can open an account online and print your labels or go to a UPS Store. Their web site will tell you the cost and exactly how many days it takes from your zip-code to the resort's...and they say it's guaranteed (check out this link:
https://wwwapps.ups.com/ctc/request?loc=en_US). We usually plan for our package to arrive 2 - 3 days before we do.( It's faster than most people think, it only takes 3 business days from Boston to WDW for example)
Also a good way to format your address label:
Hotel Name
street address
Guest Name:.......... Arrival Date:...........
reservation #
City, State, Zip
On the return you can either print your return labels and bring them with you(don't forget tape) or ask the hotel to ship it for you. If you have your own label you still give it to the front desk, you just tell them it's prepaid