Mine started in a 55gal.
At the 7 to 8 in mark she got upgraded to a 6ft 125gal
And there was a very noticeable difference in her activity level. In the 55 she just kinds floated around. In the 125 she was activity swimming around, exploring.
I got her to around 12 inches before I gave her to a fellow MFKer due to my liquid rock water.
She continued to grow and last I heard she had pushed out to 14 inches
Be prepared for many big water changes. As you have seen already I am sure Oscar's are very messy eaters, with half the food flying out of the gills...
Remember the simplest solution to pollution is dilution. At the 12 in size Oscar's poo is as big around as a pencil and 3 to 4 inches long. So as they grow you will need to increase the frequency of water changes. Or just start it now like I did with water changes every three days, down to fin level to make sure you have a strong healthy fish.