ok, I have not answered this yet but I will share my thoughts with you, maybe we can nut it out together.
The first thing I thought of it that I want the highest possible value of a_1.
Each number is different so the smallest factor possible is 2
so if I let the the first one be A then the numbers will be
A,2A,4A,8A,16A,32A,64A
If A is 1 that is ok
If A is 2 then 64A is too big so the first term has to be1
The multiple could be 2 every time giving one possible sequence.
What about if one of the muliples was 3 rather than 2 then it would end in 96A, So there are more possibilities there.
What about if one of the multiples 4 rather than 2 then it would end in 128A which is too big
so now you have a lot less possibilities to consider.
Can you find the answer and tell us what it is ?