Approximately how big is the campsites for D-loop? Also is there signal to use the internet?

Wifi is rubbish but 4g is good there. Lots of really big sites the reservation guide has recommended max vehicle sizes. It's a good campground.

D-loop sizes vary. Try Reserve America and search for Camp Ground by the Lake for a Not To Scale Map. You can also try to call the campground and ask them if you have a specific need in size. It is owned by the city of South Lake Tahoe. Google that to find their number. Wi-Fi speed varies depending on your location in the park and demand. On Holidays you can count on little to no service.

In not sure about the size of the sites, however there is wifi, it just might not be strong enough to reach you. You would have to go closer to the front gate.