You can now play Dota 2's newest and silliest mode in the beta Reborn client, if you're so inclined. The 10 vs 10 all-star mode wasn't meant to be officially released until later this week, but some enterprising individuals pounced on the download button when it leaked and have now reuploaded it. A quick stab of the subscribe button should get you what you need.
It isn't quite as simple as just being expanded team sizes. Both Silencer and Bloodseeker are banned for power level reasons. Bloodseeker's movement speed scales in accordance with how many enemy team members are on low HP. It doesn't have a natural limit, so this would put him somewhere around the speed of sound in a 10vs10 game. Silencer just has abilities that are too strong when combined with another nine players instead of four. Your opponents being unable to do anything is pretty bad for them in a 5v5, in a 10v10 it's time to say goodbye to all your health bars.
There's very, very little chance anything bad will happen to you for using the above map, even though it is unofficial. Valve will be aware of it and if they didn't want it to be on the workshop, they'd take it down. Finding a game might be challenging since it requires so many players to be online at once and there's no matchmaking for it. Once you've subbed, you'll need to launch Dota 2 in Reborn mode and head to the custom games menu.
It looks like the group matchmaking bug from last night has now been fixed, but there's been no official word from Valve so I still wouldn't risk it. In fact, anything but fun casual games and experimenting with custom modes I'd keep away from the Reborn client for now, unless you specifically want to help test it.