A three-dimensional finite element solver is applied to investigate the performance of Ga2O3/GaN nanowire transistors. Experimental nanowire results of 50 nm gate length are provided to compare with the simulation, and they show good agreement. The performance of a shorter gate length (<50 nm) is studied and scaling issues of the short-channel effect are analyzed. With a better surrounding gate design and a recessed gate approach, the optimal conditions for a 20 nm gate length are explored in this paper.