Slipway limits are often budget choices, not absolute barriers

Was Kaiser built on the German slip later used for Gneisenau and Graf Zeppelin, and what would reuse require?

Probably; German yards often enlarged slips in place, so reuse depends more on funding extensions and dredging than on an absolute physical barrier.

German slips were often modernized and expanded in place rather than replaced. A British wartime example has houses cleared rapidly to extend a slipway, while another option is to pour concrete into an estuary and dredge the approach. These works may be disruptive and expensive, but a short slip is not automatically proof that a power cannot build a larger ship. The important distinction is between technical capability and a government's willingness to fund the infrastructure.