The current work performs air-water experiments in three different pipe sizes of 38 mm, 51mm, and 102 mm to investigate the frequencies of plug and slug bubbles in horizontal flows. The local four-sensor conductivity probe is used to establish experimental data for large elongated bubbles. The investigated conditions focus on a relatively high superficial liquid velocity region (1.0 m/s ≤ jf ≤ 4.0 m/s), where limited experimental data are available in the literature. The investigated superficial gas velocity (jg) ranges from 0.19 m/s to 3.0 m/s. In total, 19 conditions are studied. To identify the elongated plugs and slugs, the local four-sensor conductivity probe is located near the top wall. Based on the flow visualization study, the stable plug and slug bubbles cannot be shorter than a pipe diameter. Therefore, a chord length greater than a pipe size is used as a criterion to pick up the elongated plug and slug bubbles. Using the obtained data, the frequencies of plug and slug bubbles are analyzed. For the investigated conditions, the Strouhal number (St) ranges from 0.03 to 1.68, while the St of the existing data in the literature are generally below 0.1. It is found that the frequency of elongated bubbles increases as jf increases, but first increases then decreases as jg increases. This indicates that large elongated bubbles start to coalesce once jg reaches a certain value. This threshold has a higher value of jg than the plug to slug transition point. In addition, the frequency decreases as pipe size increases. The existing correlations in the literature are evaluated using the newly collected data. It is found that the St number-based correlations can predict the data with higher accuracy. New correlations are developed using the new data and they can predict the frequency with an absolute average percentage difference of ±20%.