Dynamically changin size of colliders

You should not be changing the size of colliders if you can avoid it but doing to a handful is hardly going to cause any problems but the typical and scalable solution is to simply use physics queries to determine colliders around you. Queries such as OverlapCircle, OverlapBox, OverlapCapsule etc are perfect.

 

Firstly I didn't answer this specifically. Colliders don't get resized each simulation tick because why would they if they are not resizing. So no, there's no overhead if there's nothing done.

If you resize a collider, its geometry has to be recalculated (don't do this for a PolygonCollider2D as that can be expensive), considered if it affects the mass but ultimately ignored if it's a trigger then all contacts are calculated each step and reported via callbacks. Assuming then you also have scriptings in these callbacks which have to process the contacts to determine if these are of interested too can add up. You might find not doing that and performing queries which are very fast give you better performance and are simpler to handle but ultimately it's up to you.

Without being able to run your game or do profiling I cannot state which will actually be better for performance for you but modifying colliders can become much more costly than running simple queries. Only you can determine which is better for you however using the profiler. In the end, you might have so few that the difference is negligible.