SMALLER,,,smaller,,,defininetly. However you have now created more work. Not just shoring up the inside and insulating the space but the outside area must also be flashed.Buying custom fit windows make life much easier and you could ellect to go with the plain sashes rather than those offered with simulated grating between panes to off-set the extra cost.
The bottom sill plate is typically one of two different angles which can be determined with a cheap plastic protractor. The brochures offered at your supplier, will show you where to measure and the up-and-down measurement is critical, at the spot you measure, as I have mentioned,due to the angle of the seal.
"pella" makes a replacement sash kit which has some different sizes that come stock,,,however this system may require that you are a bit more "handy" than the average joe. The end result is a window which looks more original, in that the size hasn`t changed any. A replacement window is like putting a box inside a box,,,it will initially appear like a smaller window,because it is. In another house ,I went with the larger "cost saving "stock window, which had a drastic visual effect of a larger window, however, all the trim boards will be too short and not wide enough.You can usually always tell if a house has had replacement windows, simply by driving by and looking at the outside flashing. It mostly will appear as though the window has a large picture frame around it, this due to the ease of breaking flashing that is wide and flat rather than following the original architecture of the trim boards,,,Which is laborous and drains the profit. Making a break of 1 or 58 inch in the middle of a piece is not easily done on a break, but can be done.The results are rewarding!