I agree w/Sean, 5-1/4" is OEM.
Here's a replacement that uses CREE LED's, which are supposed to be the gold standard from what I've read. Note it requires a larger bucket.
CREE LED headlight optic approved for Yamaha V-Max 1200, gives a modern look to your motorbike. Powerful pure white lighting, dual position (high and low beam), plug and play thanks to its H4 connectors. Delivery 24H, free shipping!
www.ledperf.us
I use a 7" round headlamp and a H-D lens, w/a halogen bulb. Downtube headlamp brackets are used, I suggest using something which allows for rubber insulation in the bucket's 'ears' mounting, to help with minimizing vibration. Remember to '92 is 40mm and '93-'07 is 43mm for downtube size.
This EMGO 7" bucket is what I use:
Find the Emgo Black 7 in. Headlight - 66-64313 at Dennis Kirk. Shop our complete selection of Harley Motorcycle parts and accessories including the Emgo Black 7 in. Headlight - 66-64313.
www.denniskirk.com
The EMGO and the CREE 7" in one of their styles would be a good choice.
Someone thinking of a 5-3/4" bucket may be thinking of a Lucas-style product (British bikes)
Find the Emgo Black 5-3/4 in. Lucas Styled Side Mount Headlight Shell - 66-65074 at Dennis Kirk. Shop our complete selection of Harley Motorcycle parts and accessories including the Emgo Black 5-3/4 in. Lucas Styled Side Mount Headlight Shell - 66-65074.
www.denniskirk.com