Pamela, 

Regardless of tense - ce sera/ce serait/c'était etc, this has the same rule as for any 'stand-alone' adjective as given for the present tense example ' c'est (adjective masculine)' in the attached lesson. 

The adjective will always be masucilne in this construct.  This holds true when the adjective is 'stand-alone' but itself modified by an adverb - tellement in this case.  'Stand-alone adjective' here indicates the lack of a specific noun being directly decribed by the adjective. The distinction is covered in the lesson also, in discussing "cette jupe est chère"  compared to ' c'est cher ". 

Perhaps an " c'est adverb-adjective " example such as this could be included in the lesson or covered in a subsequent advanced lesson ?

 Standalone adjectives after c'est are always masculine

There are many instances where our answers are the same as one of the given answers but still it suggests correction. It happened multiple times. Could you please fix it ?