I found a relatively simple way to enlarge my Warforged, by using Alter Self through the Transformation domain.
On caster level 4, I can transform myself into Warforged Charger.
Can it handle my guisarme?
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Warforged Charger does have hands, and no clear indication that they cannot be armed, which means that you should be able to handle weaponry. On the other hand, their running style explicitly involves both hands and feet. It would be an entirely reasonable DM ruling to say that you can't run with things in your hands.
Transformation Domain gives you +1 caster level to your transmutation spells, so you should be able to do it now (as a third-level cleric with the transformation domain).
Alter Self has some interesting implications when going from living construct to living construct. Make sure that you're real clear on what you lose and what you gain. In particular, it says nothing at all about subtype, which is a little funny. This will likely take some DM adjudication of its own.
Worth noting that if you're using the same guisarme, wielding a medium guisarme as a large creature is going to give you a -2 penalty to hit (as well as doing a bit less damage than a large guisarme would). Having a sizing enchantment on the item (+1 equivalent) can fix this for you, but it might be cheaper to have two.