Card sizes

A6 is half of A5 (105 x 148mm) or (10.5 x 14.8) cms
A5 is half of A4 (148 × 210mm) or ( 14.8 x 21) cms
A4 measures (210 × 297mm) or (21 x 29.7) cms (standard size printer paper)
A3 is twice A4 (297 × 420mm) or (29.7 x 42) cms
A2 is twice A3 (420 × 594mm) or (42 x 59.4) cms
A1 is twice A2 (594 × 841mm) or (59.4 x 84.1) cms
A DL size is the A4 folded into 3 equal portions - it's a standard office size and dead easy to get the envelopes.

Envelope sizes are usually given in 'C' numbers

114 x 162mm or (11.4 x 16.2) cms (C6)
110 x 220mm or (11 x 22) cms (DL)
130 x 130mm or (13 x 13) cms (square)

Other common sizes that you can get are:
162 x 229mm or (16.2 x 22.9) cms (C5)
125 x 175mm or (12.5 x 17.5) cms
155 x 155mm or (15.5 x 15.5) cms (square)