"The table is long."
テーブルが長いです and テーブルは長いです both means in general tables are long or some table is long. (Note that I am not talking about universal facts, just in contrast to a particular object.)
Now, the difference between は and が in this case is that,
テーブルは長いです A simple description of a table or multiple tables. In this case "the table is long" means "some table is long." We know there is a table. The important information we get from the sentence is "long." The table is not short/thin/tall. The table is long.
テーブルが長いです It implies that a table is long, but not other things. Something is long there, not the chair/TV/carpet. The table is long. We know there are something is long and we learn that it is some table that is long.
そのテーブル makes the table a particular one vs テーブル only. The particular table can be a physically near one or logically near one, where it can refer to a table discussed earlier in the conversation. To summarize
そのテーブル can mean a particular table physically near the listener but further from the speaker. In this case, "that" table is used.
そのテーブル can mean a table introduced in the conversation, like "There is a table in the conference room. The table is long." We use そのテーブルは長いです.