I'm making a Fighter/Ranger who wields a glaive and who will have Guardian Soul, a 3rd-level feature of the Primeval Guardian ranger subclass (from Unearthed Arcana: Ranger & Rogue), part of which says:

Starting at 3rd level, you gain the ability to temporarily grow and
take on the appearance of a treelike person, covered with leaves and
bark. As a bonus action, you assume this guardian form, which lasts
until you end it as a bonus action or until you are incapacitated.

You undergo the following changes while in your guardian form:

• Your size becomes Large, unless you were larger.


• Any speed you have becomes 5 feet, unless the speed was lower.


• Your reach increases by 5 feet.

While wielding a glaive, which has the Reach property, I have a reach of 10 feet. With Guardian Soul, that reach increases by 5 (to 15).

Does the Large size I get from the transformation of Guardian Soul inherently add more reach? When I read about reach, it seemed to say that Large creatures have a base reach of 10 feet. So would that be +5 reach from Large, +5 for Glaive, and +5 for Guardian soul?

If I'm already Large and have a reach of 10 feet, and the Guardian Soul transformation just adds 5 feet to my reach, does it stack?

Being Large doesn't inherently increase your reach (from the edge of your space); you just have a larger space to reach from

In general, creature size is not directly related to reach. It allows you to control a larger portion of the battlefield simply by being larger (and thus blocking a wider area, being able to be adjacent to a greater number of enemies, etc.). Some Large-size (or larger) monsters may have a greater reach, but this does not define a general rule that applies to all creatures of a larger size.

This is supported by the answers to this question: Does enlarging a character with a reach weapon affect his threat range?

Mike Mearls, co-creator and designer of 5e, supports this on Twitter here:

Enlarge Person: Does the attack reach of the target increases with it's size?

only if the spell specifies it does - there's no inherent reach by size IIRC

In short: being Large doesn't automatically affect your reach.

Reach weapon + Guardian Soul = reach of 15 feet

Glaives have the Reach property:

Reach. This weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for opportunity attacks with it.

And the Guardian Soul feature says that when you're transformed:

• Your reach increases by 5 feet.

(This feature doesn't specify that it applies only to particular attacks, so it is assumed to apply to any and all attacks you can make while transformed.)

Taken together, these two things mean that you have a reach of 15 feet with your glaive while you are transformed into your guardian form.

Creature sizes don't have an inherent reach value.

Becoming Large, in of itself, doesn't change a creature's reach. The Creature Size rules don't specify anything beyond the space occupied by creatures of each size.

A Medium character with a glaive who activates Guardian Soul would have a reach of 15' — base 5', +5' from the weapon, +5' from Guardian Soul.