What is the form of a pastoral elegy? I would like to know the usual rhyme scheme and meter of a pastoral elegy.

There really is not a simple answer to your question, but this information should help you understand more about pastoral elegies.
An elegy is a formal poem of mourning. The word "pastoral" in literature connotes country life as an idealized existence. A pastoral elegy is a poem that mourns the death of an important person or idea and often casts the speaker and/or the subject as shepherds with the attendant Christian overtones.
In their ancient Greek version, elegies were written as couplets with alternating lines of hexameter and pentameter.
In English literature, pastoral elegies do not have a set form with regard to length, rhyme scheme, or meter. 
For an example of perhaps the most well-known English pastoral elegy, read John Milton's "Lycidas" available here: https://www.poetryfoundation.org/poems-and-poets/poems/detail/44733
In "Lycidas," Milton did not employ a regular rhyme scheme. He used alternating iambic pentameter and trimeter.
Pastoral elegies can even be written in free verse, such as Walt Whitman's elegy to Abraham Lincoln, "When Lilacs Last in the Dooryard Bloom'd."
https://www.poetryfoundation.org/poems/44733/lycidas