When someone is said to be "strong" in the Force, it means that he has a high midi-chlorian count. Qui-Gon explains what midi-chlorians do:
Midi-chlorians are a microscopic lifeform that reside within all
living cells and communicates with the Force...Without the
midi-chlorians, life could not exist, and we would have no knowledge of the
Force. They continually speak to you, telling you the will of the Force.
Qui-Gon Jinn, Star Wars: Episode I The Phantom Menace
In order words, midi-chlorians translate between the Force-sensitive individuals and the will of the Force. In fact, that is exactly how they are described in the Legends novel Darth Plagueis (it isn't canon, but I find the explanation informative):
A common misconception held that midi-chlorians were Force-carrying particles, when in fact they functioned more as translators, interlocutors of the will of the Force.
Darth Plagueis, p. 17
A person with a high midi-chlorian count is better able to communicate with the Force. Imagine you are trying to communicate with someone in a foreign language -- the bigger your vocabulary in the foreign language (akin to more midi-chlorians), the easier it is to communicate your desires in that language. Therefore, a person "strong" in the Force is better able to exert his will through the Force and lift heavy objects or similar.
The Force itself, however, is an energy field:
The Force is what gives a Jedi his power. It's an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.
Obi-Wan Kenobi, Star Wars: Episode IV A New Hope
The Force is so powerful that it literally binds the galaxy together. The Force itself can lift any object -- in that sense, "size matters not". Anyone who can communicate well enough with the Force can move any heavy object. However, this is easier for someone strong in the Force.
Consider another analogy: imagine you are trying to program a machine or robot to perform some work that no human is strong enough to do. For example, suppose you are programming the trash compactor on the Death Star. The trash compactor is like the Force and is able to exert forces no human can. The programming language used to control the trash compactor is like the midi-chlorians. The better programmer you are in that language, the easier it is to write a program to get the trash compactor to do what you want it to do (this is akin to being "strong" in the Force and having a high midi-chlorian count). A person who does not know the programming language at all (i.e. not Force sensitive) will find it impossible to program the trash compactor properly.