Lucky you I had an L shaped one covered in roses until the hurricane 1987.
This is what the RHS say "With larger pergolas a common practice is to plant two climbers near the base of each pillar, for example, a strong-growing climbing rose with a less vigorous clematis or honeysuckle, the rose providing additional support for the other climber. Strong-growing climbers such as Clematis montana ‘Rubens’ or wisteria will, in time, cover cross-beams and probably need some restriction".

In planting terms I agree with Drc but I am not sure what you mean by " How much space at the base of the pillars"

I presume and would expect, that the pillars are built on concrete bases, and the concrete will be at least 3-4" wider all round the pillars so this will determine the planting distance from the pillar/ s.

To take Drc's comments further I would plant the robust plants on the out facing side and the clematis on the inside facing surface as they like their roots in the shade.

You would then be required to train the clematis runners round the pillar to mingle with the more robust plants.

In terms of space you would have to plant tight into the concrete base/s.

Just a word of warning! do not plant ericaceous plants next to the concrete as this area might be a bit too alkaline.

The hole size will be dependant on the plant types you use but I would say 12"-18 square would be a fair average.

I hope that helps...Tg