I'm currently playing the Icewind Dale: Rime of the Frostmaiden published adventure. My Bugbear has been affected by Vlagomir's spark (see area H30 on p. 226-227), and he has grown to be 21 feet tall as a result. He now has the a strength of a Frost Giant (23).
My Bugbear was previously 7 feet 11 inches.
Are there rules to suggest that the reach of my bugbear should increase to 15 or even 20 feet?
UPDATE: I talked to my DM and he made a ruling that I was happy with that made logical sense. He now considers me a Huge creature with a reach of 15 feet (20 feet with my glaive) on my turn. He said that if my height is changed then it also makes logical sense that the length of my arms has changed. He considered the 25ft I was hoping for but started thought that was too much. My height tripled, logically the length of my arms should have tripled. If my proportions are all the same so my reach on my turn anyway should triple. I'm happy with the extra 5ft even though I wanted 10. He could have said no extended reach at all so for me this is a compromise I'm happy with.
Bugbears get Sneaky. In addition, without squeezing, you can move through and stop in a space large enough for a Small creature. Since I am now Huge, he is going to allow me to move through space one size smaller which is Large. His logic which I agree with is that Small is one size smaller than Medium and using RAW on the ability doesn't make sense. Otherwise, I'd still be able to move through a space large enough for a Small creature.