Just to be clear in BOTH British and US English a "kilo" is the commonest, correct and usual translation of the SI unit kilograme (kg.) See definition A2 in the Cambridge dictionary. It is not slang and is not regarded as an abbreviation. It is the number 1 definition in Merriam Webster.
For non-native English speakers just to say we buy Eggs by number and size - eg. a dozen large eggs.