What is the range of feedback resistors of LM2577 adjustable version?
The typical application from the datasheet shows feedback of a few k\$ \Omega\$.



Digging a little deeper for an underlying reason, look at the input bias current on the feedback pin (which is attached to the error amplifier)



At the maximum possible current, a resistance in the 100s of K\$ \Omega\$ or M\$ \Omega\$ would give a significant error from the feedback network; keeping the values below a few 10s of kohms minimises that error.

I would use resistors in the range of a few k\$ \Omega\$ for this.