You can get the screen dimensions using
Dimension dimens = driver.manage().window().getSize();
double xCoordinate = dimens.getWidth() * 0.5;
double yCoordinate = dimens.getHeight() * 0.5;
//you may have to cast these values to int
Using these values in a tap for example:
driver.tap(1, xCoordinate, yCoordinate, 800);
will cause Appium to tap in the middle of the screen using one finger for 800 milliseconds.