Unless the exact value of font size is very important, you can use the features of the
caption package. (If it is, see Old answer below.)
Go to Document --> Settings --> LaTeX preamble, and add
If you want to change all the captions in your document to a larger font with added linespacing, add the following line to the preamble as well:
or if you want it to apply to tables only,
caption allows you to customize your captions a lot, see the manual for details.
If you want to apply this to just a single caption, add an ERT within the table/figure float, before the caption, and write the
I'm not sure why you want this, and I wouldn't do myself, but if you're doing this for a single caption, you can add an ERT at the beginning of the caption containing
To change the font size as well, use
The second argument to
\fontsize is the baselineskip, set to 1.2 times the font size.
This does not, however, change the size of Table 1. To change that you can use the
caption package, as explained above, and use