Valmiki Ramayana
Book VI : Yuddha Kanda - Book Of War
Chapter [Sarga] 28

Introduction
Shuka in his turn enumerates the enemies and completes the account given by Sarana.

सारणस्य वचः श्रुत्वा रावणम् राक्षस अधिपम् |
बलम् आलोकयन् सर्वम् शुको वाक्यम् अथ अब्रवीत् || ६-२८-१

After listening to Sarana's words Shuka, pointing out all that army of monkeys, spoke the following words to Ravana the Lord of Demons.

Verse 2 to 32 one can read in spare time

शतम् शतसहस्राणाम् कोटिमाहुर्मनीषिणः || ६-२८-३३
शतम् कोटिसहस्राणाम् शङ्कुरित्यभिधीयते |

"Wise men call a hundred lakhs as a crore. A hundred thousand crores is reckoned as a Shanku."

शतम् शङ्कुसहस्राणाम् महाशङ्कुरिति स्मृतः || ६-२८-३४
महाशङ्क्य्सहस्राणाम् शतम् वृन्दमिहोच्यते |
शतम् नृन्दसहस्राणाम् महावृन्दमिति स्मृतम् || ६-२८-३५
महावृन्दसहस्राणाम् शतम् पद्ममिहोच्यते |
शतम् पद्मसहस्राणाम् महापद्ममिति स्मृतम् || ६-२८-३६
महापद्मसहस्राणाम् शतम् खर्वमिहोच्यते |
शतम् खर्वसहस्राणाम् महाखर्वमिति स्मृतम् || ६-२८-३७
महाखर्वसहस्राणाम् समुद्रमभिधीयते |
शतम् समुद्रसाहस्रमोघ इत्यभिधीयते || ६-२८-३८
शतमोघसहस्राणाम् महौघ इति विश्रुतः |
"A hundred thousand Shankus are said to be one Maha Shanku. A hundred thousand Maha Shankus are called one Vrindam here. A hundred thousand Vrindas are said to be one Maha vrindam. A hundred thousand Mahavrindas are called one Padmam here. A hundred thousand padmas are said to be one Mahapadmam. A hundred thousand Mahapadmas are called one Kharvam here. A hundred thousand kharvas are said to be one Mahakharvam. A hundred thousand Mahakharvas are called one Samundram. A hundred thousand Samudras are said to be one ogha here. A hundred thousand oghas are acclaimed a one Mahaugha."

एवम् कोटि सहस्रेण शन्कूनाम् च शतेन च || ६-२८-३९
महाशङ्कुसहस्रेण तथा वृन्दशतेन च |
महावृन्दसहस्रेण तथा पद्मशतेन च || ६-२८-४०
महापद्मसहस्रेण तथा खर्वशतेन च |
समुद्रेण च तेनैव महुघेन तथैव च || ६-२८-४१
एष कोटिमहौघेन समुद्रसदृशेन च |
विभीषणेन वीरेण सचिवैः परिवारितः || ६-२८-४२
सुग्रीवो वानर इन्द्रस् त्वाम् युद्ध अर्थम् अभिवर्तते |
महाबलवृतो नित्यम् महाबलपराक्रमः || ६-२८-४३

"This Sugreeva, the king of monkeys, having great strength and valour, always surrounded by a colossal army, is approaching you to make war, accompanied by the valiant Vibhishana and the ministers, as also a hundred thousand crores of Shankas, a thousand Mahashankus, a hundred Vrindas, a thousand mahavrindas, a hundred padmas, a thousand Mahapadmas, a hundred kharves, samudras and Mahaughas of the same number, and a crore of Mahanghas wholearmy as such is identical of an ocean."

This is how huge Bhagwan Ram's Army was and it is believed that Ravana’s Army was much larger

Now imagine the space required to move such a huge Army and then for the two Armies to fight

Even if both army stand shoulder to shoulder the current land size of Sri Lanka is not enough

So whwere is such huge Lanka which can have Crore plus armies?