A size adjustable assemblable pallet includes a central base member and multiple side base members, connecting bars, and loading plates. The central base member has a main body, which defines multiple positioning grooves that cross at a junction. Each side base member has a base body, which defines multiple fixing grooves. Each connecting bar has a main portion and multiple inserting portions. The inserting portions are bent extendingly in a same direction from respective ends of the main portion. The inserting portions are inserted into the fixing grooves of two side base members or one of the fixing grooves of one side base member and one of the positioning grooves of the central base member, in order to secure the central base member and/or side base members. The loading plates are secured to the side base members or the connecting bars.