I am new in string theory and recently I came up to the D-branes. I have understood all the mathematics and the technical definitions, that is, they are the (p+1)-dimensional objects upon which open strings (with p spatial Neumann boundary conditions) attach their endpoints etc. However I have not understand the intuitive part of the story.
We can imagine the string as a 1-dimensional object with a very small length, which we have not yet observe directly because -theoretically- it needs a big amount of energy. Ok with this.
However, how can we imagine the D-brane in the spacetime? How much is it extended?