In terms of length, the flip flops were designed with a reasonably ‘snug / tight’ fit in mind. If you go too big in terms of length, the orthotic does not sit in the correct position on the foot. It is important to note, that it is a concern if some of the back edge of the heel hangs over the edge of the flip flop (as per the below image).
Of course, if your heel is completely over the edge of the heel cup and squashing down on the rim of the flip flop, then that is different, however, some heel overhang is ok. Again, the most important thing is that the flip flop is comfortable and the orthotic sits in the correct position on the foot.
Please see the image below of acceptable overhang.
Please see the image below of too much overhang.