If Catalonia Were To Go...

Spain's numbers would change with the departure of Catalonia.
'It would mean the loss of 25-30% of Spain’s gross domestic product (GDP), says Spain’s Minister of the Economy, Luis de Guindos. And that’s something the government “will never let happen.”...'. Quote taken from Wolf Street here.
Tourist figures would have to be reconsidered. 17 million of the 72 million international tourists that visited Spain last year went to Catalonia for their holidays.
The size of Spain itself would shrink from 506,000 square kilometres to 474,000 km2. The population would shrink from 46,400,000 to around 38,900,000, and with the departure of Catalonia and its average GDP per person of $33,580 per year, the Spanish production, currently at $26,650 would fall as well.
The Catalonian independence groups, currently running the government there, want a referendum on secession from Spain to be held in September, or, they say, they might just declare a unilateral state of independence beforehand (maybe in August, when Madrid in on holiday).
From the figures above, it's not likely that Madrid will allow any of this to happen.