This doesn't feel to me like a CSS question as much as it's about changing requirements and CSS... or more specifically, requirements traceability in the context of CSS...
That will apply accross the board for all tables. Now, what if they change the requirment so that some tables are 600px.
That requirement won't just say "some tables are 600px wide" and if it does you need a better way to elicit requirements.
Instead it would probably be something like "tables on the HR 'staff directory' page will be 600px wide." Make the CSS rule reflect that specifically!
body#staff-directory table {
width:600px;
}
... or "tables of search results will be 600px wide.":
table.search-results {
width:600px;
}
You might roll your eyes and think "But then I have so many similar CSS rules!" but they've already changed their minds once, so don't be surprised when they do it again!
Those 'redundant' rules will come in handy when the client does change the requirements again and says "Tables on the HR 'staff directory' page will be 600px wide; tables of search results will be 800px wide and all other tables are 500px wide."
Now those crummy, non-descriptive CSS attributes "Size1" and "Size2" have shot you in the foot.