Lot Size
The standardised quantity of an instrument you buy or sell in a single trade.
In forex, a standard lot is 100,000 units of the base currency. A mini lot is 10,000 units, a micro lot 1,000, and some brokers offer nano lots of 100. Lot size is the dial that sets how much each pip of movement is worth to you.
Position sizing works backwards from risk, not forwards from ambition. Decide the maximum you are willing to lose on the trade - commonly 1% of the account - then divide that by your stop-loss distance in pips and by the pip value to arrive at the lot size. The market decides the outcome; you decide the exposure.
Brokers offering micro and nano lots are far friendlier to small accounts, because they let you keep risk proportionate instead of forcing an oversized position. If a broker's minimum trade is a full standard lot, a $500 account cannot be traded responsibly there.