One way to measure the volume of any irregular object (in your case, a stone) is to submerge it completely under water and measure the change in the height of the water level. This change in the water level (let's say it goes from 50 mL to 65 mL) indicates that the stone has a volume of 15 mL. Remember that 1 mL = 1 cm³ so you can also express the volume of the stone in cubic centimeters. However, this method requires the stone to fit inside the beaker, so perhaps you can locate a beaker large enough for your stone to fit in.

Another way to determine the volume of the stone is possible if you know what its density is (Density = mass/volume). Different materials have different densities, but if you know the type of stone that you have, it might be possible to find out this information. In that case, you could calculate the volume by determining the mass of the stone (in grams by placing it on a scale) and dividing by the density.

You are right about measuring the volume of a stone by measuring the amount of water it displaces. If you were to calculate the volume using a ruler (length x width x height), it would not be accurate due to the irregularity of the stone.To measure the volume of a stone that is too big to fit into a beaker, perhaps you could just find a bigger water pitcher from the store that has graduated marks telling you the volume. Alternatively, you could use a hammer and maybe a chisel to break the stone into smaller pieces that will fit in the beaker (you should wear safety glasses and probably should have an adult help you). The total volume of the stone would be equal to the sum of the volumes of the pieces.