1 nano-lot = 100 units of currency
1 micro-lot = 1,000 units of currency
1 mini-lot = 10,000 units of currency
1 standard lot = 100,000 units of currency
2,500 units of base currency means: 25 nano-lots, or 2.5 micro-lots, or 0.25 mini-lots, or 0.025 standard lots.
On most trading platforms, you are not able to trade in fractions of a lot.
So, for instance, if you have a MICRO ACCOUNT, in which the minimum position size is 1 micro-lot (1,000 units of currency), you will have to trade in INCREMENTS of 1,000 units. You can trade 2,000 units (2 micro-lots), or 3,000 units (3 micro-lots), but not 2,500 units (2½ micro-lots).
If you calculate an allowable position size of 2,500 units of currency, and have to choose between 2,000 units or 3,000 units, go with the smaller position size.
Some trading platforms (Oanda, for instance) let you trade in units, rather than lots. On that type of platform, you could open a 2,500-unit position.