Your premise that Kenneth Branagh's adaptation of Hamlet is an epic must be proven; an examination of the film will determine whether your premise is true. Though the movie does not meet the requirements for a classical/traditional epic, an examination of three less specific definitions of "epic" will help establish whether or not this film qualifies for that title.
The first definition is "surpassing the usual or ordinary, particularly in scope or size." While it is not elaborate or extraordinarily ornamental (no glittering thrones or diamond-encrusted tiaras, for example), the setting of Branagh's Hamlet is grand and expansive. The castle is large and the grounds are substantial. Perhaps compared to other movies, Branagh's Hamlet is not particularly epic; however, compared to the other movie versions of Hamlet, his film could qualify. Unlike, say, the more rustic and sparse royal residence in the 1990 version of Hamlet, this is an estate for which it might be worth committing murder.
Another definition of "epic" is "heroic; majestic; impressively great." This does not particularly apply to setting but could be applied to plot. Branagh's film is quite long, something which may be acceptable in live theater but is not as well received in move theaters. To ensure that the audience is not bored, Branagh has to be impressive. He does this by using dramatic color schemes (all red, white, and black) and by using familiar actors to play many of the characters. He also uses flashbacks to create a more impressive film. He uses flashbacks with the Ghost to make it clear that Claudius is a usurper and with Ophelia to reveal his point of view about her relationship with Hamlet--they had one and it was serious. Because of these impressive elements, this film could be considered epic.
A final definition of "epic" is this: "an episode in the lives of men in which heroic deeds are performed or attempted" This could be applied more to theme than either setting or plot. In fact, this definition could be a tag line for this play: Hamlet attempts--and finally performs--a heroic deed. To be completely fair, this has more to do with Shakespeare's story than to Branagh's interpretation; however, it is Branagh himself who makes this theme believable in his movie. We see and feel Hamlet's anguish and doubt as he struggles to act; his soliloquies are particularly impressive (and true Hamlet fans are thankful that Branagh did not edit them for time or to "dumb them down," as other film versions have done).
Using these definitions of "epic," we can agree that Kenneth Branagh's film version of Hamlet qualifies because of the setting he chose, the plot techniques he utilized, and his effective presentation of the play's theme.