Currently there is yes, though as I understand it thats a performance limit placed in and has been known to change where the limit is. FrostByte have mentioned they want large capital ships too. In the future I expect it to go up slightly, and with beam welding implemented the other day, In theory it already did go up just then via fewer bolting joints consuming that limit.
Someone with CA access will have to correct me for the following, but AFAIK there is nothing stopping you from bolting/welding together 'segments' at the moment to make something larger than max size, because AFAIK only the editor has size restrictions placed on it? This line is speculation, someone with CA access, or FrostByte, will have to confirm or deny this, and this is based purely on my observations of others playing.