Spacing around operators and relations in math mode are governed by specific skip lengths. Just put small before the equation and normalsize after it if you want to shrink the font but its usually better to use an ams multi-line equation environment than to change font size. Single formulas must be seperated with two backslashes.
In certain cases it may be desirable to include normal text within an equation.
Embed formulas in your text by surrounding them with dollar signs. Single formulas must be seperated with two backslashes. Thinmuskip by default it is equal to 3 mu medmuskip by default it is equal to 4 mu. Open an example in Overleaf.