[MRM]

Since you have existing openings and they are not a "standard" size, you will need custom-sized windows for your openings. There are two excellent options you should consider. First, as mentioned, Andersen makes an excellent product. Their main line brands are all made for standard size openings, but they have a custom line for existing homes called Renewal by Andersen. Renewal by Andersen is made for projects like yours. You would call them directly to get a quote.

The other option is Marvin. Marvin makes a similar wood window but their windows are all custom sized, so you can use any of their brands. They have a broad range of wood, fiberglass and vinyl. You can call any local Marvin distributor to get a quote.

EDIT: I didn't see that the reason for replacement was that the frame bottoms are rotting. This means one of about three things. First, check with Pella to make sure the windows aren't defective. From time to time window manufacturers get batches of wood preserver that doesn't work, or wasn't applied correctly, etc. I don't know of any issues like that with Pella, but it's a possibility.

The second and third issues are more serious. You may have a water infiltration issue with your house. Since the house is 30 years old and you haven't noticed anything else, it seems unlikely that you have a significant water infiltration issue, but you should not have rotting windows, even at 30 years old. Have you done anything to check the rest of the wall to make sure it hasn't been damaged? You might be getting water in around the perimeter of the window where the frame meets the siding or cladding. Make sure the perimeter of the window is caulked where it meets the house.

The third issue that comes to mind is condensation that comes from humidity in the air condensing onto the relatively cool surface of the window glass and then draining onto the frame. What level of humidity do you keep your home? It needs to come down as the outside temperature gets colder.