Lots at South Point average 75' x 90 '. This is pretty standard building lot size for the country of Belize, not only the island.
Regarding infrastructure, you better get in touch with the GOB (Government of Belize) and BEL (Belize Electricity Limited).
There is an assigned road area in the South Point development, however, it 'goes nowhere', as it has to pass through another private property to connect with the public access road at the airstrip. The owner of the private property currently says "NO" to the idea.
It is stated that BEL will bring electricity to the area when it has collected $40K US to pay for it. The South Point development encourages buyers to put a $1K US into a 'kitty fund/escrow' for this purpose.