These are the recommendations of the setspace package:
If a different spacing is required [than
\singlespacing,\onehalfspacing, or\doublespacing] then the\setstretch{baselinestretch}command can be used in the preamble to set the baselinestretch appropriately.
\baselinestretch scales the value of \baselineskip, and it is generally recommended not to change the value of \baselineskip itself.
So say that I need to set my document with an 11pt font with 12pt line spacing. Rather than just telling LaTeX to use 12pt spacing, the "recommended" way is to perform some math to figure out the appropriate factor of \baselineskip, like this:
\documentclass[11pt]{article}
\usepackage{setspace}
\setstretch{.88235}
\begin{document}
\the\baselineskip % gives 12.00002pt
\end{document}
Clearly there must be a better way? I'm reminded of the \fontsize{size}{skip} command, where you could just say \fontsize{11}{12}, but as far as I know, this is intended for temporary changes to the font size and line spacing, and not to be used as a preamble setting for the entire document.
\fontsize{size}{skip}command for a temporary font size change, but nothing like that for a permanent change. – Sverre Aug 28 '15 at 12:57\fontsizeis permanent if you do not enclose it in a group, but setting\normalsizeis, as you see, more than just setting the default font and baselineskip, it makes sense to specify other dependent lengths at the same time. Of course you do not have to do that but then when some figure or whatever issues \normalsize to get into a standard state, less things are reset. – David Carlisle Aug 28 '15 at 13:00size11.clo(those withdisplayshortskipetc.) should not be affected by changing the linespacing? – Sverre Aug 28 '15 at 13:06\fontsize{11pt}{12pt}\selectfont...the text would go back to 11pt but any equations would have lots of space,\normalsizetries to ensure everything put back to normal. – David Carlisle Aug 28 '15 at 13:10bprather than the TeX pointpt. You probably assumed that the people who want me to set these point sizes have big points in mind? I don't think they have any idea about the difference:)– Sverre Aug 28 '15 at 13:25