Hi @sree.raghu143,

You can increase the font size of site title and tagline with few lines of custom CSS through Theme Options.

Just go to Appearance->Customize->Custom CSS in your Dashboard and put following CSS code.

h1.site-title{
	font-size: 35px; /* make sure you change this value as per your requirement to change site title */
}

h2.site-description{
	font-size: 18px; /* make sure you change this value as per your requirement to change tag line*/
}

Hope this helps resolving your problem.