What exactly is a lot size?

Hi Guys, I know this has been asked a million times but how do i calculate a lot size exactly? I know its a unit of measurement that measures the amount of the transaction blah blah blah, but how do i calculate it correctly? I have been told to trade with a maximum of 0.3 lot for every 10K i have in my account with a 100-1 leverage, so what would the actual monetary value of my lot be in this case? Thanks in advance....

Its 100000 units of the base currency, so 0.3 lots would be 30000 units.

For example a long Eur/Usd trade assuming a rate of 1.5850 would translate to:

+30000 Eur / -47550 Usd (1.5850 * 30000)


Margin requirement assuming 1:100 would be $465,50 (47550 * 0,01) or 300 Eur if you have an account in Euro.

Babypips has a great explanation about it I suggest you review there some.