When you develop your web design, you will have to convert from a .PSD (Photoshop) file into an HTML file or browser readable file. Remember that the sizes of the fonts is not the same as the browser size fonts.
According to Sherri Fleming, “the font size conversion visual chart only works for fonts as they appear on the computer screen in a browser window. The font size conversions will not likely translate to print nor translate to other programs as other programs will have their own default font size and thus could potentially start from a bigger or smaller font as default throwing off the comparison charts for that program. Case in point, Photoshop will display point sizes differently than a browser will. Thus designers designing for the web using Photoshop will need to know that specifying PTs to developers will not be pleased to find their font sizes will be off once the page is in a browser”.
To give you an idea of the font sizes in browser translation, please refer to this link for the table.