Size exists only in the mind. There is no objective way to determine size. This is what Berkeley was talking about.
You're begging the question here. How do you know that size only exists in the mind? The answer is, you don't know - you can only assume that it does, and that is why it is begging the question.
To a human, a basketball is small. To an ant, a basketball is HUGE. What is the objective size of the basketball? Does the basketball have 2 different sizes at once? It's all based on perception, which is based on the mind.
Nonetheless, the basketball exists. We can measure its circumference, and that will not change regardless of who measures it, or what they measure it with. Only the units used to measure it will change. Its actual size does not.
Same thing with color. A person sees something as completely red and you see it as completely blue. Does the object have 2 different colors at once? No, because color only exists in the mind.
So close and yet so far. It's true, our perceptions of color are based on the sensory data that our brain takes in from our eyes, which means that someone with colorblindness will perceive less colors than someone without. But the actual frequencies of light don't change depending on who sees them. If you take the light frequency at 4 x 1014
Hz and show it to various people, they may perceive it as different colors, but the frequency itself will not change.
None of our senses escape the problem of perception, which is why things can only exist in minds. Ergo, God's mind is why everything exists.
Thanks to this, I've managed to pinpoint yet another contradiction in your reasoning. You see, logic is based on perception too. Your mind is totally dependent on what you perceive, and any logic you use is totally dependent on the reasoning processes within your mind - which are informed by what you perceive. So, the fact that you cannot escape the problem of perception means that your conclusion - that things can only exist in minds - is inescapably false. It doesn't matter whether you're talking about a human being or a brain in a jar - it is still dependent on sensory inputs to provide it with perceptions that it can then use for other purposes. Without those perceptions to give it information to work with, it has nothing. It would be like a hard drive without an operating system - it wouldn't be able to actually do anything.
So, even if your supposition that God's mind is why everything exists were actually correct (which is exceedingly doubtful), he would still have to have perceptions to work with, coming from outside him, in order to be able to have anything inside his mind at all. Same thing if we ever managed to make a self-aware computer program - it would be dependent on what it perceived in its virtual environment, but without humans to have written the programming code (and thus the environment we exist in), there would be no virtual environment to begin with. In short, even God would need an environment that predated him, that he got all of his information from, in order to create an entire universe within his mind.