I am trying to relocate and install a new hall light fixture to a concrete ceiling. The electrician asked that I supply all the Wiremold material needed to run the power from the existing fixture to the relocated fixture. No problem except I don't know what box to get. The new fixture has a 5" by 1" round mounting base. I have seen Wiremold fan boxes but they are 5 1/2" in diameter and therefore not acceptable [don't want the box to show]. Would appreciate any guidance as to what type, style Wiremold box I need. Many thanks.

Wiremold comes in different sizes depending on the amount of conductors to be used.
That round box is likely the one you have to use, It is better to have it this way than Wiremold box being smaller.
To go from the ceiling down a wall will require a inside corner, around a corner will require an outside corner, They also make a twisted left, and a twisted right corner.
They also make fittings that convert conduit to wiremold.
There are pieces that cover wiremold connection points, and straps to secure.
After wiremold is up, caulk and paint can improve the look.
Have done some jobs where contractor channeled a grove for conduit, and recess the box to eleminate the surface wiring option,
If the area above fan location could be drilled down through from above, may be an option,
My least favorite is the Swag chain.

Found this:
Wiremold - Metal Raceway

Then click on parts list to see available parts and also view the installation process.

If you can work with electricity, you may want to do yourself.
The installer will Locate and mount Fan Box, then Install a box over existing switch, or a receptacle. Then measure off floor, wall, or ceiling to keep everything square when running the channel.

He will run to nearest outlet, nearest outlet may not work well, more material could be used for a better look,