How can I set a minimum width (in characters) for a TextView?

Ems works if your font is monospace. For proportional fonts, you want it the width of two '0's, not two 'm's. You can fall back to TextPaint.measureText like so:

float measureText = zip.getPaint().measureText("00000");
zip.setWidth(zip.getPaddingLeft() + zip.getPaddingRight() + (int) measureText);

try this:

android:layout_width="wrap_content"
android:minEms="2"

Best way is to use TextPaint. Here is a simple example:

TextPaint textPaint = new TextPaint();
textPaint.setTextSize(yourTextView.getTextSize());
yourTextView.setMinWidth((int) textPaint.measureText("-88.88"));

Do not forget to specify text size in your TextPaint object
In measureText enter longest possible string you're expecting to see (I've written "-88.88" assuming my text can handle positive or negative numbers below 100 with 2 digits after comma)