That is a good question. Let's revisit that after working out the basics.
SIZE: L (trunk 30'-60' tall, branches and roots up to 120' long)
Start at Huge or Gargantuan? It might be nice to go Gargantuan, as that is an underserved niche.
Ability scores of similar "tree monsters"...
Ironmaw (H): Str 30, Dex 9, Con 23, Int 4, Wis 13, Cha 14
Octopus Tree (H): Str 30, Dex 3, Con 20, Int 8, Wis 15, Cha 15
Orcwort (C): Str 39, Dex 7, Con 29, Int 10, Wis 16, Cha 8
Dark Tree (H): Str 28, Dex 8, Con 21, Int 5, Wis 10, Cha 7
Ancient Night Twist (H): Str 45, Dex 6, Con 34, Int 12, Wis 16, Cha 23
Flesh-Mulcher Tree (H): Str 20, Dex 10, Con 20, Int 2, Wis 10, Cha 4
Bonetree (H): Str 10, Dex 10, Con 20, Int -, Wis 10, Cha 2
Death's Head Tree (H): Str 20, Dex 6, Con 18, Int 3, Wis 5, Cha 1