The shrinking of the "2" in $\rm{H_2O}$ is governed by the option ScriptSizeMultipliers
which is set to 0.71. Setting this to 1 leaves its size unchanged. You can set it as an option to Style
, or in the Option Inspector (Format menu or Ctrl+Shft+O) click through to Formatting Options > Expression Formatting > Display Options > ScriptSizeMultipliers
and set it at the level you want (Selection/Selected Notebook/Global Preferences).
Another option is to edit your notebook's style sheet. Under the Format menu, click Edit Stylesheet...
. In the sheet that opens click on the link "Default.nb". In the sheet that opens next click on the link "Core.nb". In this sheet, open the "FormatType styles" cell and copy the "TraditionalForm" cell. Paste this in the first style sheet. Select the cell, and press Ctrl+Shft+E (Show Expression). Add these two options:
SingleLetterItalics->False,
ScriptSizeMultipliers->1
press Ctrl+Shft+E again, and from now on $\rm{H_2O}$ in an inline equation should display in this notebook as you want:
.
Perhaps the spacing could be improved somewhat, but that's another question.