What do you intend to use your new greenhouse for? If you are not overly ambitious and just want to grow a few tomatoes and cucumbers, a small greenhouse is enough. Even a small lean-to greenhouse would suffice unless you have a large family. Tomato and cucumber plants are grateful if they get enough water and fertilizer.
However, if you plan to grow more crops and intend to make the greenhouse your hobby, you must be careful not to buy a small greenhouse. A larger greenhouse provides many more opportunities to grow various vegetables, fruits, spices, etc. The plants must have "air" around themselves to reduce pests and diseases. For example, try with pepper, spice and peach. Lettuce become extra crispy, if grown in a greenhouse. You can also prepare and grow summer flowers for your the garden in your greenhouse - so consider you needs and dreams.
You will find as your greenhouse hobby evolves that the opportunities in terms of interesting plants to grow are almost endless.
Also consider if you want to relax with a cup of tea inside the greenhouse close to your plants and your garden. If this is the case you should buy a greenhouse of at least 9.9 m2. If your greenhouse is to be used as an extended living room you should invest in safety glass. This provides extra safety in case of glass breaking, and it provides a clearer view to your garden.
Consequently, a rule of thumb is: Do not buy your greenhouse too small. The larger the greenhouse, the more room for both plants and greenhouse owner - and it will keep you satisfied and happy in the years to come.