Federal courts can issue opinions on any issue for which an appellant has standing. They tend to rule only as broadly as necessary to decide the particular case or controversy in front of them. Federal courts can also deny appeals. For both of those reasons, they cannot be forced or made to define the limits of reasonable search.
However, the US Supreme Court has clarified many times whether a particular behaviour constituted a search, and if so, whether it constituted a reasonable search.
In Katz v. United States 389 U.S. 347 (1967), the court held that attaching a listening device to the exterior of a telephone booth constituted a search, and that it was an unreasonable search.
In Smith v. Maryland 442 U.S. 735 (1979), the court held that using a pen register to obtain the records of phone numbers that were dialed by an individual was not a search.
In United States v. Jones 565 U.S. ___ (2012), the court held that installing a GPS on Jones's car was a trespass and per se an unreasonable search.
In Heien v. North Carolina 574 U.S. ___ (2014), the court held that a seizure and subsequent search is reasonable if based on an officer's reasonable mistake of law.
There are many more.
As an example of the U.S. Supreme Court explicitly avoiding a broader ruling than necessary to decide the case before them, see United States v. Jones:
It may be that achieving the same result through electronic means, without an accompanying trespass, is an unconstitutional invasion of privacy, but the present case does not require us to answer that question.
It was possible to decide the case based solely on the trespassory test, so the majority didn't go further.