How much acid to use for stabilizing meringue?
As there are multiple types of meringues, I imagine the core of this question is really the effect of acid on egg white foaming.

According to KATERYNA LOMAKINA and KAMILA MÍKOVÁ writing in the Czeck Journal of Food Science, there is a moderately complex relationship between pH of the egg whites, and its foaming capacity (overrun) and the stability of the foam:


  By the addition of a small amount
  of 1N H2 SO4  or NaOH to  the liquid egg white (pH values: 9.5, 8.6,
  6.3, 4.7, 
  3.1, 1.0), NAKAMURA and SATO (1964b) obtained  a great foaming capacity at the neutral and acidic  pHs except at the exceedingly
  acidic pH (pH 1.0).  The foam stability was high at pH 8.6, the pH of 
  the natural egg white, and decreased with changing pH. 
  
  With an aqueous
  egg albumen solution, HAMMERSHØJ and LARSEN (1999) established that 
  the foam overrun was the highest at pH 4.8 and  the lowest at pH 10.7.
  The foam stability against  drainage was the best at pH 7.0 after 30
  min, but  on a long-term scale the foam at pH 4.8 was the  most
  resistant to drainage. This is the result of the  more rigid behaviour
  of the surface at pH 4.8 and  the formation of small bubbles,
  therefore a slow  drainage of liquid from the foam, lower dynamic 
  surface tension causing the high overrun. 


The conclusion is that you will get the most volume of foam at moderately acidic pH around 4.8, but the best stability of the foam once created at the alkaline pH of egg whites.

The moral seems to be that for stable egg foam, not to add acid.

Later in the article, they also indicate that volume can be increased without ill effects by adding up to 40% more water to the egg whites.

Also, adding sugar to the egg whites inhibits foaming, requiring more agitation to form the foam.