A treaty based on ship size rather than fleet totals

If the powers cannot agree fleet ratios, could they freeze current programs and limit only the size and guns of future ships?

Yes; the likely compromise is a qualitative treaty with individual ship limits, probably shaped by G3 and N3 construction, but no cumulative tonnage cap.

A failed ratios negotiation could still produce a qualitative treaty. The powers might limit the maximum displacement and gun caliber of each ship while abandoning cumulative fleet tonnage. Because negotiations would take longer, Britain could continue the G3 battlecruisers and perhaps begin the N3 battleships before a ceiling was agreed. Their roughly 50,000-ton scale and 18-inch armament could then become the accepted maximum. Britain and the United States would preserve their advantage by building as many ships as their economies and yards could support rather than by receiving a larger formal ratio.