The scale of the platform in comparison to Skaia
Shown only in A2 and B2 so far, the victory platform is what appears beneath
Skaia at some point during a session, presumably only those destined for success in creating a new universe. The platform is metallic, according to Karkat, and consists of three thick lily pad shaped rings stacked on top of each other, with the symbolic image of a frog on the top of the platform, surrounded by a wide spirograph logo. Beneath the platform is a glowing sphere with the image of a narrower spirograph. It is unclear what triggers the platform to appear, or indeed whether it simply exists throughout the duration of the session, as a platform without an Ultimate Reward doorway has been shown.
The Ultimate Reward
The Ultimate Reward is what awaits players who have successfully won a session of Sburb. After the necessary actions for winning have been performed, a structure resembling the session's Sburb logo appears on the victory platform. The structure has a door with a smaller version of the respective Sburb logo on it, with a doorknob resembling a dream bubble, adorned with the same narrow spirograph. The door serves as an entrance into the new universe the players have created, and is apparently unusable if it has been structurally damaged.
What, if anything, the Reward includes beyond access to the new universe is currently unknown, as none of the five sessions shown in Homestuck have managed to successfully claim it. The A2 trolls came closest, as they were able to achieve victory, but were prevented from getting their Reward when the B1 Jack Noir arrived in their Incipisphere and destroyed the door before they could open it. The A1 trolls and B1 kids were unable to win and had to scratch their sessions. The B2 kids are stuck in a void session, but it became winnable post-retcon when the B1 kids arrived and the Condesce made Jade deposit their Battlefield into Skaia. Caliborn's dead session is unable to produce a new universe, so its victory and defeat conditions (and presumably its Ultimate Reward) differ from that of a normal session.
It is implied here that the reward for a normal session would include ruling over the created universe. However, it could differ between sessions, and it's possible that the implication was more about Karkat's intentions than the reward itself.