The size of your pergola will determine a number of things; the overall weight of the structure, how much support it will require and how much you’ll need to spend on materials. The bigger the pergola is, the more work will be involved in construction and the more it will cost, so keep the budget in mind when planning. You will need to account for the weight of any roofing materials if you’re planning on having a covered pergola, and possibly the weight of any plants like creepers or vines that might be trained to grow over the structure if you plan on keeping the roof open.
Post and footing requirements
A pergola of a larger size will require more support and that means more posts. The weight of the beams, joists and any cladding will all be resting on the posts so your site must have adequate clearance for support. The posts must be able to be dug in with adequately planned foundations, or you're in for a lot of heartache.
The spacing of the posts should ideally over-compensate for the weight of the roof to allow for ageing of the structure and weakening of the materials, as well as the shifts in weight that might come with things like rainfall or the collection of leaf litter. Where additional support is needed, you may need to include some bracing - particularly for overhanging roofing which can place an uneven amount of pressure on key support posts.
The sorts of calculations necessary to ensure that a pergola's adequately designed (particularly a larger pergola) will typically fall to a structural engineer.
House support
If your pergola is attached to your house, it is vital that it has enough support or you may risk doing a great deal of damage to your home. The attachments to the house should preferably be in direct contact with your house's frame, using extended support beams if necessary. Attachments to the outside veneer of a house should be solid, frequent and well fastened to evenly distribute the weight and reduce the load on any single attachment. There’s no point to having a beautiful pergola that will tear holes in your back wall next time it rains heavily.