Size it to your real work plus margin, because bigger costs rigidity and money. For RC wings, a 1000–1300 mm wire span with ~300–400 mm of X/Y travel covers most panels. For architectural letters, signs and props, you may want 600–1200 mm tall cuts. For packaging/inserts, smaller and stiffer is better.
Remember the wire span (tower spacing, the Z dimension) is set by your frame, while X (depth) and Y (height) are your travels. Longer wires sag and bow more and need more tension and heat, so do not over-build the span. A 4-axis machine around 1000 × 300 × 300 mm is a sweet spot for hobby use.
Remember the wire span (tower spacing, the Z dimension) is set by your frame, while X (depth) and Y (height) are your travels. Longer wires sag and bow more and need more tension and heat, so do not over-build the span. A 4-axis machine around 1000 × 300 × 300 mm is a sweet spot for hobby use.