If you're not willing to completely throw away a piece of work the moment something goes wrong, it's too big.
The bigger the slice of work, the more deeply you are invested. The more deeply you are invested, the less willing you will be to abandon work. The less willing you are to abandon work, the more that work must be "right" up front. The more you have to be right up front, the more timid you will be in selecting your changes. Since you so carefully pick your changes, you'll want to make sure they are big enough to be "worth the effort".
It's a vicious cycle that destroys any sense of safety and, consequently, and kind of speed.
There's no real harm in cutting past the point where you'd be willing to throw everything out when it goes wrong. It just means you'll throw out less when you find something did go wrong. In fact (you'll be surprised to discover on your own), you'll make fewer mistakes and have to discard fewer changes in the first place.
If you're looking for the right size of an increment of work, try "atomic". That is, try a slice of work that is so small it cannot possibly be divided into two smaller, complete efforts.
Failing that, get as close as you can by continuing to ask yourself this question:
How can I do less of this, now, and the rest of it, later?
