When Albus Dumbledore finally reaches the island with Harry Potter and finds out that you can't get near the potion, he made a cup and drank it. Why couldn't he just have poured the potion on the "floor"? Why couldn't he have made a big cup to hold almost everything inside the basin?
It was impossible to pour it on the floor. It was majicked to only decrease from drinking. Quoting from HP6 (Chapter 26: The Cave)
... "But how to reach it? This potion cannot be penetrated by hand, Vanished, parted, scooped up, or siphoned away, nor can it be Transfigured, Charmed, or otherwise made to change its nature."
Almost absent-mindedly, Dumbledore raised his wand again, twirled it once in midair, and then caught the crystal goblet that he had conjured out of nowhere.
"I can only conclude that this potion is supposed to be drunk."
"What?" said Harry. "No!"
"Yes, I think so: only by drinking it can I empty the basin and see what lies in its depths."
The reason for having it set this way was for Voldemort to incapacitate the would-be thief so he could be interrogated further:
"... he would not want to immediately kill the person who reached this island," Dumbledore corrected himself. "He would want to keep them alive long enough to find out how they managed to penetrate so far through his defenses and, most importantly of all, why they were so intent upon emptying the basin. Do not forget that Lord Voldemort believes that he alone knows about his Horcruxes."
"Undoubtedly," he said, finally, "this potion must act in a way that will prevent me taking the Horcrux. It might paralyze me, cause me to forget what I am here for, create so much pain I am distracted, or render me incapable in some other way..."
Of course, for such incapacitation to occur, it is imperative that the poison be actually drunk, as opposed to safely disposed of.
As far as making a bigger goblet - it is fairly obvious that the drinking must have been done using this specific goblet:
Water could not be touched in a regular way (
He met an invisible barrier that prevented him coming within an inch of it)
Later, Harry tries to fill the goblet with water with regular spells and that didn't work - which likely means it was a specific goblet magicked to not contain any water other than from an Inferi-infested lake. This is a wee bit speculative since that could have instead been a property of the entire cave instead of the goblet.
He was getting old anyway so he decided he should be the one who drank the potion so Harry and himself could recover the locket so it could then be destroyed.