When I try to measure an element I get really big numbers, I tried to measure by using in the attribute table -> open field calculator -> output field type (whole number (integer)) ->row_number - Geometry - area$
An important aspect to consider when measuring spatial data is what units the coordinate reference system being used is measured in, e.g. degrees (a geographic coordinate system), or metres (a projected coordinate system)? Often when unexpected results come up, it is because your data is measured in degrees (it may be using a spatial reference like EPSG 4326) rather than metres of Easting and Northing (e.g. defined using a UTM grid zone like EPSG 32755, or EPSG 3857).
Also worth considering is, if you are measuring area and you are using a projected coordinate system with units of metres and were expecting a result in square kilometres or hectares, the result in square metres will be much larger (1 sqkm = 1,000,000 sqm; 1 hectare = 10,000 sqm). So, you may have to scale your result to get the expected magnitude for your purpose.
A quick way of measuring in QGIS is using the measure line or area tool, described here.
I'm not sure if these considerations reflect the issues you are facing.