Double hung window installation

I have 2 questions:

1. What makes for a good window installation?

2. Is it feasible to have a window with the dimensions 42" wide by 60" high in a double hung style? The bottom would be 42 x 24 and the top would be 42w x 36h. I had an installer tell me it would be too heavy to operate as a single hung. Thanks, Bill

The size is no problem as far as structural concerns with most well built windows.
It is the weight of the sashes and whether or not the counterbalance system used in the window can handle the weight over time that is the concern of manufacturers. If the counterbalances are sized correctly, the size you state should be no problem.
If the counterbalances cannot handle the weight, then some will suggest going with single hung, or going with a double hung and field fixing the top sash.
Of note, block and tackle balances systems usually can hold a little more weight than the more popular constant force.
Some manufacturers can also upgrade to a commercial balance if the construction of the profile allows it.