Ok so there seems to be a difference in IUI/OI and IVF. In IVF they want slightly smaller follicles, but in IUI/OI they don't seem concerned with 20 odd mm follicles.
This doctor from the Homerton talks about it a bit here:
'Standard practice in most assisted reproductive technology clinics is to trigger ovulation for intrauterine insemination (IUI) when the lead follicle is 1.8cm in diameter or greater. The present study examined almost 1000 IUI cycles receiving either clomiphene citrate (CC) or letrozole for ovulation induction and suggests that the clinical pregnancy rate is higher when the lead follicle is 2.3 or 2.4cm in diameter and endometrial thickness is 9mm or greater. In IUI cycles stimulated with CC or letrozole, optimal pregnancy rates may be achieved at follicle diameters greater than 2.3cm and with endometrial thickness greater than 8mm.'
facultyopinions.com/prime/713947814
'For both CC and letrozole, higher pregnancy rates were achieved when the leading follicles were in the 23 to 28 mm range. The optimal size of the leading follicle was not statistically significantly different between cycles using CC or letrozole. However, for each endometrial thickness, the optimal follicular size of the leading follicle was different. Each additional millimeter of endometrial thickness increased the optimal follicular size by 0.5 mm. Thicker endometrial lining led to a higher probability of pregnancy.'
www.fertstert.org/article/S0015-0282(12)00250-6/fulltext
So maybe all is not lost @Kay00, some sources are happy with that size. It does make me think about what day I will start.
On cycle one I started day 3, I had 1 x 22mm by day 11
On cycle two I started day 2, I had 1 x 24mm and 1 x 17mm (I checked, it was 24 and not 27 last month, I lied!) by day 10