Arrowana

A quick search for Callas bikes only showed models that have square taper cranksets, so I am going to assume yours is as well.

If you have a crank puller, you can remove your current crankset, then you can measure the length of the bottom bracket spindle. If your crank arms are held on by bolts that thread into the spindle, simply measuring how long it is in millimeters will give you the length. If your spindle has threads that stick out and the crank arms are held on by a nut, then you don't include the threads when measuring the length. You should be looking for a number between 103mm and 127mm. Once you know how long it is, then you want to find a crankset that uses the same length of spindle. If the maker of the crankset has a website, that would be the best place to look. On a low cost bike like yours, you might be able to get away with a slight mismatch, if your new crankset is designed for a spindle that is <5mm shorter or longer than your current one, you might be able to make it work. Past 5mm, you are likely to run into issues like chainrings hitting the frame if it is too short, or chainline issues if it is too long.

If the crankset and cassette or freewheel are in need of replacement, you should grab a new chain as well. A worn chain on new chainrings and cogs will likely skip, and will wear down your new parts quickly.