Smaller lens is with lower focus distance which means strong laser shooting. Bigger lens is with higher focus distance which means weak laser shooting.
Usually you could choose 150*150mm lens as standard, good for most applications. Add a 70*70/100*100mm lens as spare if you do deep engraving a lot. Add a 300*300mm lens as spare if you need big size, also helpful for tumblers.
250x250mm Lens can do Deep Engraving with 50W Fiber Laser?