There is luggage storage facilities at Barjas airport:
http://www.airwise.com/airports/europe/MAD/MAD_08.html
Left Luggage facilities are available in two areas, but there are no lockers inside the terminals.
The larger of the two facilities (370 lockers) is at ground level opposite Terminal 1, Arrivals Lounge 2. Large and small lockers are provided. Passport or identification cards are required and bags are security checked.
The second facility (110 lockers) is located on the connecting walkway between Terminal 2 and Car Park 2.
Size - Period - Cost per day
Small Lockers 1 day € 2.65
2-15 days € 3.30
After 15 days € 0.66
Large Lockers 1 day € 2.65
2-15 days € 4.65
After 15 days € 1.33
Check these costs - they might have gone up this year.
The alternative is to post your excess baggage to Santiago.
Purchase a box at any post office (they are accustomed to sending on ‘paquete peregrino’). Post it to the Pilgrim Helper who provides a storage facility in Santiago where it will be kept for up to 60 days for a small fee. The name of the facility is Pension Badalada.
http://www.badalada.es. Tel (+34) 981 572 618
Address it as follows:
Pension Badalada
ATT:
(Your name here)
Rua Xelmírez, 3
15704 Santiago de Compostela
To keep the box they charge:
Small box (317 x 215 x 125 mm) €15 for 60 days
Medium box (390 x 290 x 190mm) €20 for 60 days
Large box (590 x 390 x 390mm) €25 for 60 days