Moving across periods on the periodic table from left to right, the atoms tend decrease in radius size because each atom has more protons than the atom before it, meaning there is more of an attractive force in the nucleus that pulls the electrons closer to the center. Even though there are both more protons and electrons, the attractive force of the protons wins out and makes the radius of the entire atom smaller.
The radius of an atom is not a well-defined measurement because of electron properties. At any given point, the electrons can jump closer or further away from the nucleus of the atom based on the amount of energy in the atom. Generally, there is a trend that moving along a period, or row, of the periodic table from left to right, the atoms tend to have smaller and smaller radii.
While moving from left to right across a period, the atoms are gaining protons and electrons. While it might seem counter-intuitive that adding protons and electrons actually makes the atom smaller, it is important to keep electrical charges in mind. Protons are positively-charged particles that are packed into the nucleus and attract electrons. The more protons packed together in the nucleus, the greater the pull on the electrons is, which makes the electrons come closer and makes the radius of the atom smaller.