We're not given the size of the locusts, but they were much larger than ordinary locusts.

          These locusts do not feed on vegetation like ordinary locusts, but were forbidden to hurt the grass or the trees or anything green.

                    They were to afflict only men and they had human intelligence because they afflicted only men who didn't have the seal of God in their foreheads. They were not permitted to kill, only torment for a limited time, five months.