All SMF versions.
Open ./Sources/Subs.php
Find:
array(
'tag' => 'size',
'type' => 'unparsed_equals',
'test' => '[1-9]\]',
'before' => '<font size="$1" class="bbc_size">',
'after' => '</font>',
),
Replace with:
array(
'tag' => 'size',
'type' => 'unparsed_equals',
'test' => '[1-9]\]',
'before' => '<span style="font-size: $1" class="bbc_size">',
'after' => '</span>',
),