Originally Posted by SteveCallas
Most real movie posters are 27 x 41 ish
Yup. A 1:1.519 ratio (width:height)
Most movie poster images available online are 1:1.5 ratio and good quality ones are 24 x 36.
I've worked on so many poster images over the years for people here and there are benefits to 24 x 36 from a contruction perspective. Out of a single 8 ft piece of 1x_ lumber for the frame you can build two 36" sides and one 22.5" top or bottom, or four 22.5" tops/bottoms. The waste on the lumber is minimal.
Others like bumping up the image width to 25.5" (which usually yields a 38.25" height) so that they can use the full width of their 24" wide insulation. Any smaller, like in the 24x36 example above, and you have to trim the sides, any wider, and the insulation doesn't stay in place. You then have to either buy longer pieces of 1x_ lumber to compensate for the larger sizes, or have a lot more wasted material.
I suppose if money was no object, these could be sized up to 27" x 40.5" (again, since most files are in a 1:1.5 ratio) and then you could cut the insulation into 25.5" x 24" pieces and put one in the bottom (sideways so to speak) and then a second one that is 25.5" x 15" and use it to fill the top sideways (or cut one piece down to 39" tall x the stock 24" wide, and then another piece 1.5" wide by 39" tall). Many people have tried to piece together small strips like that and it seems that the bigger piece just pushes the littler one around, thus the reason I would recommend the sideways option.
All of that cutting up of insulation should be done with care since any insulation is really bad if breathed in, but most people don't want the hassle of all of the cutting and all of the waste, and thus the reason that many choose 25.5" x 38.25" or 24" x 36".