If you change the mask size and distance before you check for any collision, it shouldn't glitch. I don't understand what you mean with that they look much bigger than the standard one. If you want Sonic to be enlarged with a factor 2, meaning that you double the height, you simply enlarge the masks gradually until they are twice as big and move them outwards until their distance to the center of Sonic is twice as long.
[Edit] You should also make so that the enlargement stops if both the bottom and top sensors are colliding with an obstacle. Same thing goes for the left and right sensors.