The length if the implants is determined by how much bone you have present. A good consideration is to have enough implants built upon that if one implant fails, there are enough implants left which will prevent the failure of your whole implant reconstruction. The cleaning demands vary depending on the type of implant reconstruction that you had. You definitely need to clean daily around the implants. Brushing with a toothbrush, as as well as flossing are important.
Electric toothbrushes can also be helpful for cleaning around implants.
Other cleaning aids may be recommended where the specific needs of your case require them. It is important to not only have a professional cleaning done around the implants, but you also should have periodic implant check-ups with your implant surgeon regarding the health of the implants. Cases do vary and only your implant surgeon can properly and accurately discuss the considerations of your specific case.