Don't worry about about making your space wider, just taller. A common beginner mistake is trying to cover too big of an area for the lights they have. Use this as a guide: 400 watts = 4x4 feet max; 600w = 5x5; 1000w = 6x6 - and that's pushing it. For optimum lighting it should really be 400 = 3x3; 600 = 4x4; 1000 = 5x5 - you get the idea ... too big of a space means lost lighting. If you keep your space to the right size you can 'contain' the most light to maximize it for plant growth. (Make sure you have excellent ventilation.)
Since you're only growing one plant you'll need to grow to a decent height to get a decent yield, which obviously means, decent height in your space. Is there a reason you're only growing one? Alternatively you could grow a bunch of smaller plants and reap (at least) the same yield. The sea of green approach offers a quicker turn around time (time to get to harvest) and (in my own opinion) it is easier to maintain healthy plants. I'll attach a couple of pics of my current grow to give you an idea. These were taken a week ago, something like that. Just trying to provide you with some useful information before you get started.
Note about the pics: These were taken on the day I put them into bloom so the males haven't been removed yet. When they are, (and as the plants get bigger) the spacing will work out very nicely, i.e., a 'sea of green' with just females. I know because this isn't the first time I have done this - the first time was in 1974.
Notice the compact growth - tight node development, bushy plants ... this is the result of proper lighting in the right sized space, as opposed to those tall stringy plants you see all over this site. (Strain makes a difference too of course, for example, sativas are stringy by nature, but with good lighting and so forth you can at least keep that to a natural minimum rather than growing some skyscraper with 6 leaves.
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