Size is not the problem — one-sided play is
A large dog and a small dog can play beautifully together, and two same-sized dogs can be a danger to each other. What matters is whether the play stays mutual. Healthy sessions look loose and silly: bodies curve, mouths hang open, the dogs swap who chases and who rolls over, and both pause on their own to sniff or shake off. Play that needs your intervention is one-directional — the big dog does all the chasing, mouthing, and pinning, while the small dog's body gets lower, tighter, and quieter, or she starts hiding behind furniture and people.
Use consent checks instead of guessing
Every few minutes, gently hold the bigger dog back for two or three seconds and watch the small dog. If she bounces back in, shakes off, and re-invites play, the session was genuinely fun for her. If she moves away, hides, or uses the pause to leave the room, the play was something she was enduring — end it kindly and give both dogs something calm to do. Run the same check in reverse if the small dog is the relentless one; plenty of terriers bully giants, and a big dog who cannot escape a pestering playmate eventually answers with his teeth.
Build the room around the smaller dog's exits
Play in spaces where the small dog can always get out of the game: a gap under a gate she can slip through, a sofa or platform she can jump to that the big dog is not allowed on, and no dead-end corners where she can be trapped and trampled. Keep sessions off slippery floors where a heavy dog cannot brake, and remove the toys that trigger wrestling over objects. Outdoors, a long line on the big dog during the early weeks of the pairing lets you interrupt a chase before speed and weight combine into a collision.
Cap the arousal before it peaks
Roughness climbs with excitement, so interrupt early and often while both dogs are still listening: call them apart every minute or two, reward a few seconds of calm attention, then release them back to play. The session should end before anyone is panting hard, barking in faces, or ignoring your voice — a good rule is to stop while you are still thinking it is going well. Separate the dogs to rest after play rather than letting them wind down on top of each other, because tired dogs have short fuses and most play fights start in the over-tired final minutes.
When to stop the pairing altogether
If the small dog starts avoiding rooms the big dog is in, if play regularly ends in yelping or snapping, or if the size gap comes with a big dog who fixates on the small one the way he would watch prey, keep them separated and bring in a qualified behaviour professional to assess them in person. Some pairings are managed for life rather than trained into friendship, and accepting that early protects the smaller dog.
Consistent refereeing is what makes mixed-size play work, so put the consent-check habit, session limits, and safe-room setup where everyone can follow them — KinsLe's Care Teams records those play duties against both dogs' profiles, each marked done by whoever supervised the session.