Two soldiers, one very small, one very tall (5)
I believe the answer is:
giant
'one very tall' is the definition.
(very tall person)
'two soldiers' is the wordplay.
'two soldiers' means two substitutions for 'soldier'.
'soldier' becomes 'gi' (GI can mean a US soldier).
'soldier' becomes 'ant' (soldier is a type of ant).
'gi'+'ant'='GIANT'
'one very small' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for giant that I've seen before include "Extremely tall person" , "Large mythical figure" , "Colossus" , "Jumbo" , "Extremely large (person)" .)