It doesn't need to be codified any more exact than the term "tramp freighter". While historically those tended to be ships on the smaller size as large ships were operated as liners, it's a term defined by activity, not size. If your ship isn't operating to a schedule, it's a Tramp.
Wikipedia (Tramp trade):
"Today, the tramp trade includes all types of vessels, from
bulk carriers to
tankers. Each can be used for a specific market, or ships can be combined, such as oil or bulk carriers, to accommodate many different markets, depending where the ship is located and the supply and demand of the area. Tramp ships often carry their own gear, such as booms, cranes and derricks, in case the ports they use lack suitable equipment for loading or discharging cargo."