You don't say which version of SMF this is for...

If it's v2.0.2, and this is a pure guess. So, keep a copy of the file, as a backup, in case I've bolloxed it up, OK?
In the default theme's directory, find Register.template.php
It looks, to me, like the " maxlength="25", in the first line, is what you're after. Dunno what the size=30 thing does, though.
<input type="text" name="user" id="smf_autov_username" size="30" tabindex="', $context['tabindex']++, '" maxlength="25" value="', isset($context['username']) ? $context['username'] : '', '" class="input_text" />
<span id="smf_autov_username_div" style="display: none;">
<a id="smf_autov_username_link" href="#">
<img id="smf_autov_username_img" src="', $settings['images_url'], '/icons/field_check.gif" alt="*" />
</a>
</span>
</dd>
<dt><strong><label for="smf_autov_reserve1">', $txt['email'], ':</label></strong></dt>
<dd>
<input type="text" name="email" id="smf_autov_reserve1" size="30" tabindex="', $context['tabindex']++, '" value="', isset($context['email']) ? $context['email'] : '', '" class="input_text" />
</dd>
<dt><strong><label for="allow_email">', $txt['allow_user_email'], ':</label></strong></dt>
<dd>
<input type="checkbox" name="allow_email" id="allow_email" tabindex="', $context['tabindex']++, '" class="input_check" />
</dd>
</dl>';