Trailer Wheel Bolt Pattern

The Harbor Freight trailer I am interested in comes with a 12" wheel, 5 bolt pattern. Over at Tractor Suply they sell trailer wheels with a 5 bolt patern set at 4 1/2". These wheel come in 13" and even 15" sizes. Q #1 Does it make any sense upgrade trailer wheels to a larger size if the bolt pattern is the same? Q#2 Does a larger wheel stress the axel? Q#3 Does anyone know the bolt patern size on a Harbor Freight that can carry load weight of 1800# This comes with a 12" wheel.
Hope you can help
DEE
Hope you can help
DEE