Antd has externized most of their styling variable in LESS variables

as you can see in

https://github.com/ant-design/ant-design/blob/master/components/style/themes/default.less

To be able to overwrite those variables you need to use modifyVar function from LESS

You can check more about theme updates here: https://ant.design/docs/react/customize-theme

So to your question you can check -
@card-head-font-size: @font-size-lg; @card-head-font-size-sm: @font-size-base

For example changing the font-size and font-family of the title using styled-components you can override the class like so:

import React from 'react';
import ReactDOM from 'react-dom';
import 'antd/dist/antd.css';
import './index.css';
import { Card } from 'antd';
import styled from 'styled-components';

const StyledCard = styled(Card)`
  .ant-card-head-title {
    font-size: 10px;
    font-family: cursive;
  }
`;

ReactDOM.render(
  <>
    <StyledCard title="Default size card" extra={<a href="#">More</a>} style={{ width: 300 }}>
      <p>Card content</p>
      <p>Card content</p>
      <p>Card content</p>
    </StyledCard>
  </>,
  document.getElementById('container'),
);

According to the docs,
You can also provide a ReactNode for the title prop, and style it as you want.

You also can check this sandBox:
https://codesandbox.io/s/basic-card-antd-4-18-5-forked-hfdnm?file=/index.js