Hi,
HikaShop has no limit on the number of characters for custom fields.
MySQL has a limit of 4,294,967,295 bytes for them though. But you won't reach that whatever text you're adding in your custom field.
However, a while ago (long before HikaShop 3.0.0), we added the support of Joomla content tags to custom fields of the type "WYSIWYG".
That way, plugins of the group "content" can replace tags in the text you enter there.
So thi can potentially take a huge toll on the processing based on what content plugins you're using.
To check if it comes from that, disable all the plugins of the group "content" and see if it displays fine.