Go Marvin they can do specialty sizes very easily. They Have double hungs called MAGNUMS, and custom sizes in other styles. Really a great window.

Ok,i find a contractor -a friend which i didnt even know he is in windows bussiness for the last 10 yrs, so he came out gave me lowest way lowest estimate than other bigger outfits or mfg instalers, and he took me to factory Advanced Windows [Chicago ],i was pretty impressed with the product ,is vynil strong sturdy picture windows ,so i vent and ordered the job ,so this friend came out today to od proper and measuring ,and quess what we find behind the cappinng outside and casing inside [old job was a aluminum windows, top pictures and botom pictures and slider,anyway what we find that the whole thing was sitting on botom 2x6 mounted slanted about 10-13%, and sides and top nothing touching only the street side 2x4 locking the windows against it and the inside casing but above and sides of windows nothing is holow space, so the friend said we have ot make a header and sides with 2x6 or 2x 8 againsdt brick and then install the windows in it ,and the sizes are pretty much will stay maybe 1/2" less, but it will be a strech 16 feet by 6feet and 12 feet by 6 feet openning and 3rd in other room 105"by 61".
I told him ,before he orders the windows, maybe we should take old out completely and build the new framing out the and see what size to orderwindows, so we dont get to small or too large ones ,then we eat it if not fit,right? They coudl probably he said build in 1 day. but he said ,hel not go that route, hel built the frame according the window sizes hel bring even hel ferezes over... [I will have those above sizes in up posting in these openning.

Vinyl stinks I recommend against it. It conducts heat, cold and moisture. Ive heard good things about INTEGRITY's ULTREX line and Marvins pull truded aluminium which is thicker and makes enough contact with the frame to drain off incidental moisture. Ask your dealer to see a corner sample of these windows.

Its pretty much standard to build your openings according to the window sizes. You could prolly save some money if you adjust the sizes a bit so you can just get standard sized new windows.

You have to take anything mlloyd says with a grain of salt, as he works for Marvin.
You'll have no problem getting units to fit your openings. Contact some local contractors, and run your ideas by them. Make sure that structurally, the walls won't need additional support. Draw out you ideas, and shop it around. Most companies can accomodate the combinations you have in mind, so don't limit yourself to just a couple.