How to calculate linear Y rod length for a larger replacement bed?
Basically, your setup is the following:



The overhang of the bed, assuming the bearings are in the center, equals (300-105)/2 = 97.5 mm on each side. So the distance from the leftmost bearing face (when bed is at y = 0 mm) to the center of the Y rods assembly equals 300 - 97.5 = 202.5 mm. Knowing this distance for the other side of the center to the right when y = 300 mm is the same, the minimum length of the rod will therefore be 2 x 202.5 = 405 mm which equals the addition of the bed length and the outer bearing distance 300 + 105 = 405 mm. 

Please note this excludes extra length for e.g. a limit switch, and some extra space around the bed. Basically the extra length of the bed 300 - 220 = 80 mm needs to attributed to each side of the bed, so 40 mm on each side extra.

For a 220 x 220 mm bed, the minimum length would be 2 x (220 - (220 - 105)/2) = 325 mm. When I measure the rods between the acrylic flanges it measures about 365 mm, about 40 mm extra for clearance around the bed (20 mm on each side).

The overhang for the 300 mm bed is not that much, and a construction under the heated plate will be used to fasten the bed at the corners, as long as that construction is not to flimsy, the current distance will be alright. For other distances you should change the value of 105 in the formulas above. E.g. for an outer flange distance for the bearings of 120 mm, you would require rods of at least 420 mm.