A clone is ready to transplant when two things are true at the same time: it has visible white roots coming out of the rooting cube, and it's showing fresh new leaf growth on top. One without the other means the plant isn't there yet.
Size matters less than root development. Most clones stand 4 to 6 inches tall when they're ready, with two to four sets of leaves above the original cut point. But a squat 3 inch clone with vigorous roots will outperform a lanky 7 inch clone with a weak root mass every time. The root ball is the engine. The canopy is just what it's pulling.
If the clone is 8 inches tall and still has no visible roots, something is wrong. It's probably either a stretch problem from too-little or too-far light, or the cut simply never rooted. Check the cube — if roots are there but hidden, you're fine. If not, read the rooting signs carefully before moving it.
Transplant into a small container first. A 4 inch pot or 16 ounce solo cup is ideal. Skip straight to a 1 gallon only if the root system is already robust. Overpotting a small clone causes overwatering, root rot, and slow growth — there's too much wet medium for a small root mass to pull, and it sits stagnant. This is also why rooted clones are easier to size up correctly: you can actually see what the roots can handle.