Try experimenting with the values in the 'Font size key' box on the Convert - Look&Feel page when you convert RTF to LRF. The relevant bit of the User Manual is
here. However, if you just want the headings to be the same size as the body text then it should be straightforward
e.g. if the setting you have in the 'Base font size' box is 12pt then set all 8 values in the 'Font size key' box to 12. As for the bold headings, if they are bold in the RTF they should come out bold in the converted book.