The other answers do a great job of covering the RAI perspective. If you'd like to push it further, there's also:
Rules-lawyer It for Maximum Cheese
The spell description doesn't say the image you create must fit inside of a 5-foot cube. It says it must be "no larger than" a 5 foot cube. Which says it's about putting the total size (or, since we're dealing with a 3-dimensional object, "volume") of a 5-foot cube against the total size (again, volume) of the illusion.
That gives you 125 cubic-feet to work with, so you can do a 7' x 5' wall so long as it's not more than ~3.5' thick. Or make it half an inch thick instead, and you can have a 7' x 425' wall, which is even better and still strictly not larger than a 5-foot cube.
I assume an infinitely large planar image (of zero thickness) would be out, however, as the description at least implies that the illusion is 3-dimensional and based upon some tangible object that you've actually observed in the 3-dimensional world. But if you're good at dividing by very small fractions of units then even taller and longer walls should be possible. It's not like illusory walls need any significant thickness to be effective, and you can probably argue that you've seen some quite flimsy/thin facade walls before.
You should be prepared for your DM to cite this answer to the question "Does “no larger than” imply shapability?" (or something like it) in reply.
At which point you'll have to do some further rules-lawyering to argue that the 5-foot cube constraint isn't describing the area of effect for the spell. You're not being asked to choose an existing area or object to work within or upon; you're told you can create a magical image of any object you like, subject to the constraints that it:
- doesn't create sound, light, smell, or any other sensory effect; and
- is not strictly larger than a 5-foot cube
A tall, long, skinny wall is not volumetrically larger than a 5-foot cube and does not create sensory effects, so it should therefore be allowed.