Every time sensible block/mass limits are suggested a loud minority of people call them oppressive and unreasonable, yet is there a server where these supposedly functional ships over 300K mass can roll up on another faction's station parked full of large ships & turrets then have another ship over 300K mass jump in, deploy fighters, and engage in battle without causing dangerous level of server instability? None I've heard of. That's a fantasy, and trying to hold out for that fantasy is killing all the MP servers ability to function smoothly at realistic levels.
If it's not going to be PLAYABLE anyway, I say keep it in SP. I think to keep things playable (not just building/designing/flying solo far from planets but actually interacting with others freely), a mass limit closer to 100K is advisable for MP where the load any one individual (no matter how spoiled and/or self-centered) puts on the server needs to be forced to respect the needs of a dozen or so other players relying on the same infrastructure. 300K is extremely relaxed; MP is about interactive play, not how big we can build. We can continue to build 500K mass giants in SP and share them here on the Dock, but they break MP servers. They just don't work and aren't playable in public MP except under very strict conditions, and therefore the server should prevent any one egomaniac from crashing it for everyone just so they can posture and primp about "how long my ship is" or how it looks. Take that stuff to the docks.
Multiplayer combat with all ships under 50K is actually mostly smooth, fun and exciting even with 4-5 players & several AI ships engaged. This cannot be said of rolling combat and dogfights with several players sporting ships over 100K with massed turrets. Very low mass limits will help servers check the docked entity capacity automatically as well, BTW because of pure surface area issues. I think the mass limit should be set very low until new performance breakthroughs are released by the developers, as should docked entity count. It sucks. We all WANT huge titans and mass carriers to work in MP, but they don't. They aren't playable. They ruin the server for others.
I'm sure there are some servers out there that would love nothing more than to have players in 500K mass ships constantly crashing the server - admins love doing all that unpaid support work so you can show off your too-big-to-fight ship - but not every server has to accommodate that nonsense. Not every admin wants to spend 6 hours a day catering to kids with toys so big they constantly break. I don't blame any admin for protecting their server from such things, because currently almost every server is plagued by lag caused largely by a tiny minority of very selfish individuals crashing around in utterly useless giants. 300K? 100K? 50K? 30K? All good - whatever works, but I say start out extremely low and raise the roof slowly once you know what works. We already know what happens with very high limits - further testing on every single server at all times is not currently valuable. Haven't seen much "alpha testing" being done on legit public MP servers with very strict size controls though.