William Pitt was never overly concerned with fashion. By no means could he be classed as a dandy or a macaroni, and he was far from obsessed with looking good and playing the part. To his credit though, Pitt did have good taste in clothing.
To get an idea of how Pitt dressed in 1798, I had a look through his tailor bills for the period. In January 1798, there were several requests issued to Pitt's clothing maker, William Morse, for the following:
"18 pairs of Callico drawers [underwear]
two striped bengall Callico dressing gowns lined through with Callico Compleat
12 fine white Quilting waistcoats broad double busted lined with Callico Quilting back Compleat
a blue cloth Corbeau coat
To making a Corbeau cloth full dress coat richly embroidered
black cloth coat silk" 
'Bengall Callico', I believe, simply means that they were white cotton men's underpants. To get an idea of what the 'bengall Callico drawers' looked like, I've included the image below:
|Fig 1: Men's drawers, c. 1805 (but very similar to what Pitt would have worn)|
|Fig 2: William Pitt by James Godby, after William Owen (c. 1804)|
In the stipple engraving shown above, Pitt is wearing what could be the 'blue cloth Corbeau coat' he had ordered to be made in 1798.
A further order to Morse made on March 31, 1798 included the following:
"fine blue & white striped bengall Callico wide & long dressing Gown lined with Callico Compleat,
six fine white Quilting waistcoats double busted Compleat,
six pair Callico drawers,
a fine blue and white Callico wide & long dressing gown,
six fine white Quilting waistcoats double busted,
twelve Callico drawers" 
Lastly, on June 4, 1798 an order for a "blue cloth Windsor Uniform Coat" was placed, followed at the end of that month by a request for "six pairs [of] fine breeches." 
|Fig 3: William Pitt as Master of Trinity House, by Gainsborough Dupont (1794)|
In the portrait of Pitt as Master of Trinity House shown above, he is represented wearing the Trinity House Uniform. The coat he requested to be made in June 1798 would have been very similar to this one, although the Windsor Uniform was more elaborate, worn primarily for state occasions, and had gold lace around the buttonholes. The breeches may have looked similar too. Unfortunately for social historians, no precise measurements for any of the garments are given. Presumably the tailor would have known Pitt's size from previous orders.
1. The National Archives, Chatham Papers: PRO 30/8/215, ff. 169-170.
2. The National Archives, Chatham Papers: PRO 30/8/215, ff. 518-519.
Fig 1: Men's drawers, 1805. Accessed from: Image Source
Fig 2: William Pitt by James Godby, after William Owen (published 1804), NPG D16446. Image Source
Fig 3: William Pitt as Master of Trinity House by Gainborough Dupont (1794). I received permission to photograph the portrait on a visit in 2012. It now hangs on the Quarterdeck of Trinity House on Tower Hill, London.