Simple answer to this is too research potential lamps you want and look at the specs of each. I'm assuming you're going LED? If so, in those manufacturers specs you want to look for their vegetation and flowering "footprint" this is the core spread of light the lamp will be able to cover. Anything outside of that footprint will get less light, or be fighting for light. Reflective sides help maintain a decent intensity at the outer edges of the tent, but it's not perfect.
Luckily, for your space, you don't need a large, expensive lamp with a large footprint. The standard, low cost LED's or quantum board LED's are built for 3x3 veg and 2x2 flowering footprint. When comparing lamps, look for an even spectrum, efficiency and output. I personally recommend Spider Farmer 1000 quantum board LED. It's top quality, new tech that covers the footprint you need. It's not very expensive and is comparible with all blurple LED's pricing but with better output and efficiencies.
Research is key here. Weigh your options and remember that lumens aren't everything. It's what spectrum and footprint those lumens can accomplish.
I wish you luck in your find. You will find something quickly, I'm sure. Look for warranties, however. Spider Farmer stands behind their products but others don't. The old saying, buyer beware, comes to mind. This will be the most important piece of your grow (aside from your actual genetics) so brand name and warranties do count here. Please don't buy a no name China brand light. They are fire hazards and they should not be in the home...trust me. Cheers👍👌!