I think these are good questions. While my answers are far from representative of all who claim to "believe" in God (and they are not always representative of my own personal views either), and I will give it a shot:
But--seriously. to those of you that believe in God: do you all believe the same thing ?
Depending on a person's religion or personal convictions, and the education that makes that up, the answer of course is "no." After leaving the Watchtower I taught Catholic and Protestant religious classes for a couple of decades. From my experience, people tend to absorb their religious training like they do their politics, based on what they desire to believe is true about what they learned about something. I know this because when you test a religion class on a subject, most of the answers that come back vary based on personal taste instead of critical analysis, no matter how much we instructors tried to teach people to employ critical thinking. People just won't do it. Humans are emotional. They claim to think like Mr. Spock but in reality approach everything like Captain Kirk.
For instance--do you believe that God is a HE ?
Jewish and Christian theology does not teach that God has any gender, though some Fundamentalists might believe that the Bible is literal.
The Jewish philosopher Maimonides from the Middle Ages taught that there was no difference between creating an idol out of wood or stone or words. It was all the same because essentially you were making a god in human form. Maimonides argued that the anthropomorphism in the Scriptures (terminology which describes God with human features and qualities) was simply word idolatry. Describing God in human form, creating a god with human feature out of words was no different than shaping one out of stone, he argued. It was not possible that the Origin of the universe could be anything like a man as early Bible writers enscribed, with arms, fingers, sitting on a throne, or even with emotions like anger or jealousy.
The early Christians, the Church Fathers, would agree, with subsequent Church councils adopting similar views. The Catechism of the Catholic itself declares:
God transcends all creatures. We must therefore continually purify our language of everything in it that is limited, image-bound or imperfect, if we are not to confuse our image of God -- "the inexpressible, the incomprehensible, the invisible, the ungraspable" -- with our human representations. Our human words always fall short of the mystery of God.--CCC 42.
Therefore it is logical to argue that God is neither a "he" nor even an entity or being, so to speak, as "God" is actually Ineffable. The language that Judeo-Christian religions use is admitedly limited due to the fact that languages have little to go on to explain what they are dealing with.
Is He invisible?
That might be a philosophical question (and not theological) dependent on who you are talking to and what they believe. But for myself, I would go back to my previous answer.
Where is He?
Again, according to theologians: God is, but whatever that means is just unknown. That "is" does not literally exist in the universe and is not "somewhere" or anywhere. Why not?
God created the universe, which is a place. A place exists in time, which was also created by God. That is what is known in theology as the "temporal." God does not exist in the "temporal" or in a place or a "where" because God is not a creation. God exists in "eternity" which is outside the temporal.
But eternity is not a place and it is not "outside" the temporal. That is why God is often spoken of as being "everywhere" or as omnipresent, because from the standpoint of the temporal in theology everything in eternity more or less has access to all of temporal at once, pass, present and future. It is as if you drew a line and you looked at it. You are over and outside the line, like "God," but we are on the line, the line representing the temporal, so to speak.
This is of course theological speak and it is far more complicated than that. However it is how theologians teach concepts regarding God and God's relation to creation.
How big is He ?
See the above.
How long has He been ( bean!) in existence ?
God exists out of time. According to theology, God was before God created time. Since time had its origin from God, then before time existed, there was God. I don't know how long ago that was.
Is He bigger than the universe ? or is He inside you ?
They may be multiple universes, which scientists believe there may be. What we call "God" does not have a "size" since that deals with place and that requires space, something that is on the temporal plane. See the above for that discussion.
Can God be inside a person? Accoding to Jewish and Christian theology, yes, in various and multiple ways. (We don't have or space to explain all of those.) Since God is obviously not limited to an entity-like person and these theologies are speaking of the origin of life, apparently "God" is in everyone.
A very ancient form of Jewish though is that there is a spark of God even in the inanimate, like rocks and trees. Some native peoples who have other types of religions believe their understanding of a creator has imprinted a spark of divinity likewise into everything as well.
