Hi,

1/ To remove the breadcrumbs, please go to Customize > General Options > and check to "Hide Breadcrumbs"

2/ To make title & body text large on mobile, please add more this code to Customize > Custom CSS:

\
@media only screen and (max-width: 479px){\
    .container-single .header-standard .post-title, .penci-page-header h1 {\
        font-size: 24px;\
    }\
    body, p{ font-size: 16px; }\
}\

Change number 24 to value you want for heading titles.

Change number 16 to value you want for body text.

3/ To change "Written by" just "By", please check options for it via Customize > Quick Text Translation

Best Regards,

PenciDesign