How would you like your garden to be? And what would it be used for? If you split it into rooms, what would you use each room for? Have you got children? What are their needs? Do they like footballing? Or hide and seek, or tree climbing? How much time have you got for gardening? Would you like to grow flowers or fruit and veg? Do you do any entertaining - do you need a barbecue area etc?
Sometimes it really helps to write down everything you want, and do a few ariel plans of the garden, adding in features in different places.
With regards to dividing it, it can often make a garden seem bigger, as the eye is drawn to the foreground and other interesting features. An opening in a hedge can create a bit of mystery. You will also have 2 aspects to consider - the view from the house towards the top of the garden, and the view from the top of the garden toward the house.
An alternative is to leave it open, but to have curved borders towards the middle which draw the eye inwards, and that can make the garden seem longer paradoxically. At the moment, the eye is drawn straight to the top of the garden, as apart from the bamboo and the arch, the only planting towards the middle is the grass lawn, which is effectively a monoculture and not the most interesting thing to look at on its own.
I'm not a garden designer btw! But I have spent a lot of time this year designing my own garden so have done lots of reading. I did decide to break up my garden with a hedge across the middle and it's great (albeit it's not as wide as your garden). I feel it makes the garden seem bigger, plus we now almost have a second garden that feels shut off from the house, with a different feel and theme to the rest of the garden.