Change font-size within a shortcode

I have a simple shortcode like this….

function myshortcode( $attributes, $content = null ) {
    extract( shortcode_atts( array(
        'class' => ''
    ), $attributes ) );
           return 'This is the content : ' . $content . ';
}
add_shortcode('my_shortcode', 'myshortcode');

This works great and if I use the following in a post…

[my_shortcode]This is the content that I want to display[/my_shortcode]

But what I want to do now is vary the font size of the content for example…

[my_shortcode]This is the [25px]content[/25px] that I want to display[/my_shortcode]

I want this to change the word ‘content’ to 25px font size.

Anyone know a way, maybe str_replace?

Are you trying to put a shortcode within a shortcode? Perhaps this is an easier solution to control your front-end output?

CSS

span.my_pixel_size{font-size:25px;}

PHP

'<p>This is the content: <span class="my_pixel_size">' . $content . '</span></p>';