i have a 150w led light. in a 90x90cm tent. and it's underpowered for this size, but, if i were to LST the plants, and always keep it under the lights, would the tent size matter? i'm talking about the reflective material. does it matter if it's 30 cm further away?
Does it matter? It may influence your grow, but... having a strong enough light for your grow and maintaining adequate conditions in the environment (heat, humidity, etc.) will be more relevant to achieve good results. Once after a move I grew in a spare room. I had sold my tent and some other supplies, so I blacked out the room, used no reflective materials other than the hood; my results were fine enough. *you always want to maximize the power of your light without overheating.
You will loss some level of reflection but all in all its better to have too big a tent than too small.