For a decent flag, it needs to be sewn. And sewn from MOD grade all-weather polyester. I'd suggest the minimum size you can get away with is a 2 yard - perfect for hanging from the roof or as a fashionable evening Pashmina for the 'Memsahib. Anything smaller is, frankly, a signal that you're a wimpish eunuch. The Dutch, of course, consider anything less than a 3-yard to be a joke. The rule for a Dutch yacht is that the tip of the ensign should be 1cm above the water when hanging limp.
Just to confuse things, Union flags come in two aspect ratios; the standard 1:2 for we wet types, and the slightly squarer 3:5 version favoured by the brown jobs. Here's a quick price comparison for (as far as I can tell) like-for-like 2-yard sewn MOD poly flags;
|Navy 1:2 vers.||Army 3:5 vers|
|91cm x 183cm||91cm x 152cm|
|Hampshire Flag Co||£54.50||£54.50|
|South Coast Flagpoles||£54.00||£47.50|
So go on. Spend £50 today on a flag you can fly (or wear) with pride ...