Step 1: Picture what rays from infinity look like.
An object at infinity sends out rays that are essentially parallel to each other and to the principal axis by the time they reach the mirror.
Step 2: Recall where a concave mirror sends parallel rays.
A concave mirror is a converging mirror, so it bends every incoming parallel ray so that they all meet at one single point, the principal focus $F$.
Step 3: Think about what meeting at one point means for the image.
If every ray from the object converges onto a single point, that point is the entire image, there is no spread left, so the image has essentially zero size.
Step 4: State the conclusion.
The image formed at the focus is therefore extremely small, that is highly diminished and point sized.
\[ \boxed{\text{Highly diminished, point sized}} \]