By the uncertainty principle particles cannot be specified in space and momentum simultaneously in the copenhagen interpretation of quantum mechanics.
If something can't be specified it does not mean that it has not exact values. A photon is a undivisible unit moving with c and it is following its geodesic path.
A bit more difficult is it if the photon is moving through an electric or an magnetic field. Since photons have an oszillating electric field component as well as an oszillating magnetic dipole moment a photon is under the influence if this fields. The photons field components are changing there sign periodically and the significance of changing the straight path of motion to a wiggling one is unremarkable for us. But in befringes materials (calcit) it leads to the deflection of light into two paths.
If photons are moving with c in every posible frame what is the spatial extent of a photon? Does it mean it is everywhere in all the space?
As I wrote on top photons are indivisible units. Of what? Of energy. Once emitted they take away from the emitting particle some amount of energy and momentum and can give this values to another particle during absorption.
In the theory of Quantum Mechanics photons are disturbtions of an overall existing electromagnetic field. This leads to same difficulties in the understanding, which you expressed in the question about the dislocation of photons all other the space. Let us see if it is possible to avoid a photon as a dislocation of all over the infinite space.
But in interference experiment photons are required to be at same time at same position to interfere.
The last question is the reason for your others? As anna v stated there are done experiments with single photons and after throwing photons one by one through a double slit the well know intensity distribution appears on an observer screen (an electronic devise or simple a photo plate). It has to be underlined that the result of some intensity distribution one get for single slits as well as for single edges!
As I told on top an bifringes material is able to split a light beam into two beams and this happens do to the photons electric and magnetic field components. But an interaction - as the expression stated - always has two players. The second player in our case are the surface electrons of the involved obstacle. So the intensity distributions behind edges are the result of the common and quantised field between the photons and the surface electrons of the edge(s). This point of view resolves a lot of difficulties.
For the question in the headline about the size of a photon see How does the size of the magnetic field vary with the wavelength of a photon?.