March size = Rally Spot base value (Lv1 800 -> Lv45 860,000) plus % buffs from other sources, all applied to that base; Free/no-spend levers: Academy research Coordination/Adv Coordination/Super/Supreme Coordination (up to +100,000 and +100%), Military Academy Siege/Melee/Charge Formation research; Rank via prestige (Knight +5,000 up to Regent +100,000); VIP4+ gives +4,000 (VIP4) scaling to +160,000 (VIP25), but requires VIP level 4+ AND an active VIP time item; General march-size skillbook Lv1-4 (+3% to +12%); generals with innate march size skill (King Arthur +15%, Genghis Khan +15%, Barbarossa +10% (Lv4 general skill only), Li Shimin +10%, others); +10-14% more from raising a general's star level to 8, +6-16% from specialty; Duty Officer assigned to Rally Spot (requires Rally Spot Lv35) up to +30%/+70,000, or Toyotomi Hideyoshi/Zhang Liang up to +21-22%; Civilization Treasure 'Imperial Seal' unlock/upgrade (Lv1 +3% up to Lv15 +74.5%); Ideal Land ornaments (+250 to +5,000 each, don't need to be placed); Citizen assigned to Rally Spot, star-leveled (Gold/Purple only), up to +48,000; Guerrilla Tactics grade breakthrough at Rally Spot (+30,000 to +160,000, ground marches only); Paid/time-limited: March Size Increase items (Junior +50%, Medium +100%, Senior +200%, all 1hr; Battlefield Super +300% for 5 min); general equipment (rings/bracers, solo +11-25%, set bonuses +10-30%); Spiritual Beast (Behemoth King etc, up to +26% at max level/quality) and Dragon (Celtic Demon +10-20%, Amaterasu Dragon Blade +15% at Lv40, Nidhogg +20% at Lv40).
Original research source: Evony Guide Wiki