Stop table going off page

\noindent\resizebox{\textwidth}{!}{%
... your tabular ...
}

In LyX insert the two lines in ERT (Evil Red Text) mode (Ctrl-L) just before and after your tabular. In the documents preamble add \usepackage{graphicx} if not already present.

This is how you add a new package: Use custom latex package in LyX

You can fix the width of the table to fit the width of your general text.

To fix the width of the table in LyX 2.0, right-click in a cell in the column, move to More... and choose Settings.... In the Table Settings tab, you can set the value near the bottom, under Table-wide settings.

This, however, will set the width of your table, but not the content of the table. If, e.g., you have a cell with a lot of text that extends beyond the margin, then also pick a width for that (those) column(s) under Column settings in the same tab. This will also allow line breaks in the text.