private static final Field sizeField;

static {
    try {
        sizeField = ArrayList.class.getDeclaredField("size");
        sizeField.setAccessible(true);
    } catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

// 'setSize'
public static <T> void growSize(final ArrayList<T> list, final int maxSize) {
    if (maxSize <= list.size())
        return;

    list.ensureCapacity(maxSize);
    try {
        sizeField.setInt(list, maxSize);
    } catch (Exception ex) {
        throw new RuntimeException("Problem while setting private size field of ArrayList", ex);
    }
}

Here is the reported issue and here a discussion. When you need decreasing the size too then have a look into Vector.setSize

A less hacky but also less efficient version would be:

public static <T> void growSizeSlower(final ArrayList<T> list, final int maxSize) {
    if (maxSize <= list.size())
        return;

    list.addAll(new AbstractList<T>() {

        @Override public Object[] toArray() {
            return new Object[maxSize - list.size()];
        }

        @Override public T get(int index) {
            throw new UnsupportedOperationException("Not supported yet.");
        }

        @Override public int size() {
            throw new UnsupportedOperationException("Not supported yet.");
        }
    });
}