Hello Alisha - welcome to the Vine!
We've been chatting a lot about this recently...if you click on 'search' at the top right of the page and type greenhouse into the box, you'll come up with loads of threads on greenhouses. Grab yourself a mug of tea ( or a glass or 2 of something more relaxing )and have a good read!
I think you'll find that you need to get the biggest you can find as you'll always run out of room inside,and they don't tend to overheat, and glass is better than plastic for light to pass through and it's a lot cheaper ( unless safety is a real issue for you?)
I have an old glass 8x6 greenhouse and have a small peach tree trained across the back and grow 8 indoor tomatoes,6 cucumber and 6 chilli plants. I don't use it for seedtrays as it is on the lottie and we only tend to go down 2-3 times a week.
Think about how you intend using it , look at the area you have available, and then (most important) at your budget.
Mine was free from a neighbour on the lottie.