speedbar-tag-group-name-minimum-length

speedbar-tag-group-name-minimum-length is a variable defined in `speedbar.el'.
Its value is

4

Documentation:
The minimum length of a prefix group name before expanding.
Thus, if the `speedbar-tag-hierarchy-method' includes `prefix-group'
and one such groups common characters is less than this number of
characters, then the group name will be changed to the form of:
worda to wordb
instead of just
word
This way we won't get silly looking listings.

You can customize this variable.